问题

What is the difference between Android units of measure?

回答1:

From the Android Developer Documentation:

px
Pixels - corresponds to actual pixels on the screen.
in
Inches - based on the physical size of the screen.
1 Inch = 2.54 centimeters
mm
Millimeters - based on the physical size of the screen.
pt
Points - 1/72 of an inch based on the physical size of the screen.
dp or dip
Density-independent Pixels - an abstract unit that is based on the physical density of the screen. These units are relative to a 160 dpi screen, so one dp is one pixel on a 160 dpi screen. The ratio of dp-to-pixel will change with the screen density, but not necessarily in direct proportion. Note: The compiler accepts both "dip" and "dp", though "dp" is more consistent with "sp".
sp
Scale-independent Pixels - this is like the dp unit, but it is also scaled by the user's font size preference. It is recommended you use this unit when specifying font sizes, so they will be adjusted for both the screen density and user's preference.

From Understanding Density Independence In Android:

+----------------+----------------+---------------+-------------------------------+ | Density Bucket | Screen Density | Physical Size | Pixel Size                    |  +----------------+----------------+---------------+-------------------------------+ | ldpi           | 120 dpi        | 0.5 x 0.5 in  | 0.5 in * 120 dpi = 60x60 px   |  +----------------+----------------+---------------+-------------------------------+ | mdpi           | 160 dpi        | 0.5 x 0.5 in  | 0.5 in * 160 dpi = 80x80 px   |  +----------------+----------------+---------------+-------------------------------+ | hdpi           | 240 dpi        | 0.5 x 0.5 in  | 0.5 in * 240 dpi = 120x120 px |  +----------------+----------------+---------------+-------------------------------+ | xhdpi          | 320 dpi        | 0.5 x 0.5 in  | 0.5 in * 320 dpi = 160x160 px |  +----------------+----------------+---------------+-------------------------------+ | xxhdpi         | 480 dpi        | 0.5 x 0.5 in  | 0.5 in * 480 dpi = 240x240 px |  +----------------+----------------+---------------+-------------------------------+ | xxxhdpi        | 640 dpi        | 0.5 x 0.5 in  | 0.5 in * 640 dpi = 320x320 px |  +----------------+----------------+---------------+-------------------------------+

+---------+-------------+---------------+-------------+--------------------+ | Unit    | Description | Units Per     | Density     | Same Physical Size |  |         |             | Physical Inch | Independent | On Every Screen    |  +---------+-------------+---------------+-------------+--------------------+ | px      | Pixels      | Varies        | No          | No                 |  +---------+-------------+---------------+-------------+--------------------+ | in      | Inches      | 1             | Yes         | Yes                |  +---------+-------------+---------------+-------------+--------------------+ | mm      | Millimeters | 25.4          | Yes         | Yes                |  +---------+-------------+---------------+-------------+--------------------+ | pt      | Points      | 72            | Yes         | Yes                |  +---------+-------------+---------------+-------------+--------------------+ | dp      | Density     | ~160          | Yes         | No                 |  |         | Independent |               |             |                    |  |         | Pixels      |               |             |                    |  +---------+-------------+---------------+-------------+--------------------+ | sp      | Scale       | ~160          | Yes         | No                 |  |         | Independent |               |             |                    |  |         | Pixels      |               |             |                    |  +---------+-------------+---------------+-------------+--------------------+

More info can be also be found in the Google Design Documentation.

回答2:

Pretty much everything about this and how to achieve the best support for multiple screens of different sizes and densities is very well documented here:

Supporting Multiple Screens

Screen size
Actual physical size, measured as the screen's diagonal. For simplicity, Android groups all actual screen sizes into four generalized sizes: small, normal, large, and extra-large.

Screen density
The number of pixels within a physical area of the screen; usually referred to as dpi (dots per inch). For example, a "low" density screen has fewer pixels within a given physical area, compared to a "normal" or "high" density screen. For simplicity, Android groups all actual screen densities into six generalized densities: low, medium, high, extra-high, extra-extra-high, and extra-extra-extra-high.

Orientation
The orientation of the screen from the user's point of view. This is either landscape or portrait, meaning that the screen's aspect ratio is either wide or tall, respectively. Be aware that not only do different devices operate in different orientations by default, but the orientation can change at runtime when the user rotates the device.

Resolution
The total number of physical pixels on a screen. When adding support for multiple screens, applications do not work directly with resolution; applications should be concerned only with screen size and density, as specified by the generalized size and density groups.

Density-independent pixel (dp)
A virtual pixel unit that you should use when defining UI layout, to express layout dimensions or position in a density-independent way. The density-independent pixel is equivalent to one physical pixel on a 160 dpi screen, which is the baseline density assumed by the system for a "medium" density screen. At runtime, the system transparently handles any scaling of the dp units, as necessary, based on the actual density of the screen in use. The conversion of dp units to screen pixels is simple: px = dp * (dpi / 160). For example, on a 240 dpi screen, 1 dp equals 1.5 physical pixels. You should always use dp units when defining your application's UI, to ensure proper display of your UI on screens with different densities.

If you are any serious about developing an Android app for more than one type of device, you should have read the screens support development document at least once. In addition to that, it is always a good thing to know the actual number of active devices that have a particular screen configuration.

Screen Sizes and Densities

回答3:

I will elaborate more on how exactly does dp convert to px:

If running on an mdpi device, a 150 x 150 px image will take up 150 * 150 dp of screen space.
If running on an hdpi device, a 150 x 150 px image will take up 100 * 100 dp of screen space.
If running on an xhdpi device, a 150x150 px image will take up 75 * 75 dp of screen space.

The other way around: say, you want to add an image to your application and you need it to fill a 100 * 100 dp control. You'll need to create different size images for supported screen sizes:

100 * 100 px image for mdpi
150 * 150 px image for hdpi
200 * 200 px image for xhdpi

回答4:

px Pixels - point per scale corresponds to actual pixels on the screen.

in Inches - based on the physical size of the screen.

mm Millimeters - based on the physical size of the screen.

pt Points - 1/72 of an inch based on the physical size of the screen.

dp Density - independent Pixels - an abstract unit that is based on the physical density of the screen. These units are relative to a 160 dpi screen, so one dp is one pixel on a 160 dpi screen. The ratio of dp-to-pixel will change with the screen density, but not necessarily in direct proportion. Note: The compiler accepts both dip and dp, though dp is more consistent with sp.

sp -Scale-independent Pixels - this is like the dp unit, but it is also scaled by the user's font size preference. It is recommended that you use this unit when specifying font sizes, so they will be adjusted for both the screen density and user's preference.

Take the example of two screens that are the same size but one has a screen density of 160 dpi (dots per inch, i.e. pixels per inch) and the other is 240 dpi.

                          Lower resolution   screen          Higher resolution, same size Physical Width                      1.5 inches                        1.5 inches Dots Per Inch (“dpi”)               160                               240 Pixels (=width*dpi)                 240                               360 Density (factor of baseline 160)    1.0                               1.5  Density-independent Pixels          240                               240 (“dip” or “dp” or “dps”)  Scale-independent pixels   (“sip” or “sp”)                  Depends on user font size settings    same

回答5:

Moreover you should have clear understanding about the following concepts:

Screen size:

Actual physical size, measured as the screen's diagonal. For simplicity, Android groups all actual screen sizes into four generalized sizes: small, normal, large, and extra large.

Screen density:

The quantity of pixels within a physical area of the screen; usually referred to as dpi (dots per inch). For example, a "low" density screen has fewer pixels within a given physical area, compared to a "normal" or "high" density screen. For simplicity, Android groups all actual screen densities into four generalized densities: low, medium, high, and extra high.

Orientation:

The orientation of the screen from the user's point of view. This is either landscape or portrait, meaning that the screen's aspect ratio is either wide or tall, respectively. Be aware that not only do different devices operate in different orientations by default, but the orientation can change at runtime when the user rotates the device.

Resolution:

The total number of physical pixels on a screen. When adding support for multiple screens, applications do not work directly with resolution; applications should be concerned only with screen size and density, as specified by the generalized size and density groups.

Density-independent pixel (dp):

A virtual pixel unit that you should use when defining UI layout, to express layout dimensions or position in a density-independent way. The density-independent pixel is equivalent to one physical pixel on a 160 dpi screen, which is the baseline density assumed by the system for a "medium" density screen. At runtime, the system transparently handles any scaling of the dp units, as necessary, based on the actual density of the screen in use. The conversion of dp units to screen pixels is simple: px = dp * (dpi / 160). For example, on a 240 dpi screen, 1 dp equals 1.5 physical pixels. You should always use dp units when defining your application's UI, to ensure proper display of your UI on screens with different densities.

Reference: Android developers site

回答6:

dp is dip. Use it for everything (margin, padding, etc.).

Use sp for {text-size} only.

To get the same size on different screen densities, Android translates these units into pixels at runtime, so there is no tricky math for you to do.

See the difference between px, dp and sp on different screen sizes.

Source: Android Programming: The Big Nerd Ranch Guide

回答7:

I have calculated the formula below to make the conversions dpi to dp and sp

回答8:

Definitions

px or dot is a pixel on the physical screen.

dpi are pixels per inch on the physical screen and represent the density of the display.

Android gives alias names to several densities

ldpi (low) ~120dpi
mdpi (medium) ~160dpi
hdpi (high) ~240dpi
- most devices in 2015 are here
xhdpi (extra-high) ~320dpi
- Apple iPhone 4/5/6, Nexus 4
xxhdpi (extra-extra-high) ~480dpi
- Nexus 5
xxxhdpi (extra-extra-extra-high) ~640dpi

dip or dp are density-indenpendant pixels, i.e. they correspond to more or less pixels depending on the physical density.

1dp = 1px on mdpi

sp or sip is a scale-independant pixel. They are scaled when the Large Text option is turned on in Settings > Accessibility

1sp = 1dp
1sp = 1.2dp with accessibility Large Text

What to use?

Use sp for Text size.

Use dp for everything else.

回答9:

Source 1

Source 2

Source 3: (data from source 3 is given below)

These are dimension values defined in XML. A dimension is specified with a number followed by a unit of measure. For example: 10px, 2in, 5sp. The following units of measure are supported by Android:

dp

Density-independent Pixels - An abstract unit that is based on the physical density of the screen. These units are relative to a 160 dpi (dots per inch) screen, on which 1dp is roughly equal to 1px. When running on a higher density screen, the number of pixels used to draw 1dp is scaled up by a factor appropriate for the screen's dpi. Likewise, when on a lower density screen, the number of pixels used for 1dp is scaled down. The ratio of dp-to-pixel will change with the screen density, but not necessarily in direct proportion. Using dp units (instead of px units) is a simple solution to making the view dimensions in your layout resize properly for different screen densities. In other words, it provides consistency for the real-world sizes of your UI elements across different devices.

sp

Scale-independent Pixels - This is like the dp unit, but it is also scaled by the user's font size preference. It is recommended that you use this unit when specifying font sizes, so they will be adjusted for both the screen density and the user's preference.

pt

Points - 1/72 of an inch based on the physical size of the screen.

px

Pixels - Corresponds to actual pixels on the screen. This unit of measure is not recommended because the actual representation can vary across devices; each devices may have a different number of pixels per inch and may have more or fewer total pixels available on the screen.

mm

Millimeters - Based on the physical size of the screen.

in

Inches - Based on the physical size of the screen.

Note: A dimension is a simple resource that is referenced using the value provided in the name attribute (not the name of the XML file). As such, you can combine dimension resources with other simple resources in the one XML file, under one element.

回答10:

Basically the only time where px applies is one px, and that's if you want exactly one pixel on the screen like in the case of a divider:

On >160 dpi, you may get 2-3 pixels,

On >120 dpi, it rounds to 0.

回答11:

Pixels - corresponds to actual pixels on the screen.

dp or dip

Density-independent Pixels - an abstract unit that is based on the physical density of the screen. These units are relative to a 160 dpi screen, so one dp is one pixel on a 160 dpi screen.

Use of dp:

Density independence - Your application achieves “density independence” when it preserves the physical size (from the user’s point of view) of user interface elements when displayed on screens with different densities. (ie) The image should look the same size (not enlarged or shrinked) in different types of screens.

Scale-independent Pixels - this is like the dp unit, but it is also scaled by the user's font size preference.

http://developer.android.com/guide/topics/resources/more-resources.html#Dimension

回答12:

Where to use what & relationship between px & dp?

Density-independent pixel (dp)

A virtual pixel unit that you should use when defining UI layout, to express layout dimensions or position in a density-independent way. As described above, the density-independent pixel is equivalent to one physical pixel on a 160 dpi screen, which is the baseline density assumed by the system for a "medium" density screen. At runtime, the system transparently handles any scaling of the dp units, as necessary, based on the actual density of the screen in use. The conversion of dp units to screen pixels is simple:

px = dp * (dpi / 160).

For example, on a 240 dpi screen, 1 dp equals 1.5 physical pixels. You should always use dp units when defining your application's UI, to ensure proper display of your UI on screens with different densities.

Understanding pixel to dp and vice versa is very essential (especially for giving exact dp values to creative team)

dp = px * 160 / dpi  MDPI = 160 dpi || Therefore, on MDPI 1 px = 1 dp For example, if you want to convert 20 pixel to dp, use the above formula, dp = 20 * 160 / 160 = 20. So, 20 pixel = 20 dp.  HDPI = 240 dpi - So, on HDPI 1.5 px = 1 dp XHDPI = 320 dpi - So, on XHDPI 2 px = 1 dp XXHDPI = 480 dpi - So, on XXHDPI 3 px = 1 dp  For example, let us consider Nexus 4. If 24 pixels to be converted to dp and if it is a Nexus 4 screen, developers can convert it to dp easily by the following calculation : dp = 24 * 160 / 320 = 12 dp Screen dimension: 768 x 1280 pixel resolution (320 ppi or 320dpi) Optional (screen size):  4.7" diagonal

Try to get all pixel values in even numbers from the creative team. Otherwise precision lose will happen while multiplying with 0.5.

px

It is explained above. Try to avoid in layout files. But there are some cases, where px is required. for example, ListView divider line. px is better here for giving a one-pixel line as a divider for all across screen resolutions.

sp

Use sp for font sizes. Then only the font inside the application will change while device fonts size changes (that is, Display -> Fonts on Device). If you want to keep a static sized font inside the app, you can give the font dimension in dp. In such a case, it will never change. Developers may get such a requirement for some specific screens, for that, developers can use dp instead of sp. In all other cases, sp is recommended.

回答13:

You can see the difference between px and dp from the below picture, and you can also find that the px and dp could not guarantee the same physical sizes on the different screens.

回答14:

Anything related with the size of text and appearance must use sp or pt. Whereas, anything related to the size of the controls, the layouts, etc. must be used with dp.

You can use both dp and dip at its places.

回答15:

I would only use dp.

There is a lot of talk about using "sp" for font sizes, and while I appreciate the point, I don't think that it is the right thing to do from a design point of view. You can end up breaking your design if the user has some wonky font size selection, and the user will end up blaming the app, and not their own life choices.

Also, if you take an sp-font app on a 160 dpi tablet, you will find that everything scales up... but your font, which is going to look tiny in comparison. It isn't a good look.

While the idea of "sp" fonts has a good heart, it is a poor idea. Stick with dp for everything.

回答16:

sp = scale independent pixel

dp = dip = density independent pixels

dpi = dots per inch

We should avoid to use sp.

We should use dp to support multiple screens.

Android supports different screen resolutions

ldpi (low) ~120 dpi
mdpi (medium) ~160 dpi
hdpi (high) ~240 dpi
xhdpi (extra-high) ~320 dpi
xxhdpi (extra-extra-high) ~480 dpi
xxxhdpi (extra-extra-extra-high) ~640 dpi

An 120 dp ldpi device has 120 pixels in 1 inch size.

The same for other densities...

We as software engineers should use this conversion formula:

pixel = dp * (density / 160)

So 240 dpi device's 1 dp will have = 1 * (240/160) = 3/2 = 1.5 pixels.

And 480 dpi device's 1 dp will have = 1 * (480/160) = 3 pixels.

Using this 1.5 and 3 pixels knowledge, a software engineer can design layouts for different densities.

To check screen parameters of any device:

DisplayMetrics metrics = new DisplayMetrics(); getWindowManager().getDefaultDisplay().getMetrics(metrics);  Toast.makeText(     this,     "4:" + metrics.heightPixels + "," + metrics.density + ","     + metrics.densityDpi, Toast.LENGTH_LONG).show();

回答17:

Difference between dp and sp units mentioned as "user's font size preference" by the answers copied from official documentation can be seen at run time by changing Settings->Accessibility->Large Text option.

Large Text option forces text to become 1.3 times bigger.

private static final float LARGE_FONT_SCALE = 1.3f;

This might be well of course vendor dependent since it lies in packages/apps/Settings.

回答18:

dpi -

Dots per inches
Measuring the pixel density of the screen.

px - pixel

For mapping screen pixels

pt - points

About 1/72 of an inch, with respect to physical screen size.

in - inch - with respect to physical screen size(1 inch = 2.54 cm).

mm- milimeter - with respect to physical screen size.

sp - scale-independent pixel.

Based on user`s font size preference.
Font should be in 'sp'.

dip -

dip == dp
Density independent pixel.
It varies based on Screen Density.
In 160 dpi screen, 1 dp = 1 pixel.
Use dp except the text font size.

In standard, dp and sp are used. sp for font size and dp for everything else.

Formula for conversion of units:

px = dp * ( dpi / 160 );

Density Bucket -> Screen Display => Physical Size        => Pixel Size                     ldpi         -> 120 dpi          => 0.5 x 0.5 in         => 0.5 in * 120 dpi = 60x60 px     mdpi         -> 160 dpi          => 0.5 x 0.5 in         => 0.5 in * 160 dpi = 80x80 px     hdpi         -> 240 dpi          => 0.5 x 0.5 in         => 0.5 in * 240 dpi = 120x120 px    xhdpi        -> 320 dpi          => 0.5 x 0.5 in         => 0.5 in * 320 dpi = 160x160 px    xxhdpi       -> 480 dpi          => 0.5 x 0.5 in         => 0.5 in * 480 dpi = 240x240 px   xxxhdpi      -> 640 dpi          => 0.5 x 0.5 in         => 0.5 in * 640 dpi = 320x320 px

回答19:

Here's the formula used by Android:

px = dp * (dpi / 160)

Where dpi is one of the following screen densities. For a list of all possible densities go here

It defines the "DENSITY_*" constants.

ldpi (low) ~120dpi
mdpi (medium) ~160dpi
hdpi (high) ~240dpi
xhdpi (extra-high) ~320dpi
xxhdpi (extra-extra-high) ~480dpi
xxxhdpi (extra-extra-extra-high) ~640dpi

Taken from here.

This will sort out a lot of the confusion when translating between px and dp, if you know your screen dpi.

So, let's say you want an image of 60 dp for an hdpi screen then the physical pixel size of 60 dp is:

px = 60 * (240 / 160)

回答20:

Screen Size in Android is grouped into categories small, medium, large, extra large, double-extra and triple-extra. Screen density is the amount of pixels within an area (like inch) of the screen. Generally it is measured in dots-per-inch (dpi). Screen density is grouped as low, medium, high and extra high. Resolution is the total number of pixels in the screen.

dp: Density Independent Pixel, it varies based on screen density . In 160 dpi screen, 1 dp = 1 pixel. Except for font size, use dp always.
dip: dip == dp. In earlier Android versions dip was used and later changed to dp.
sp: Scale Independent Pixel, scaled based on user’s font size preference. Fonts should use sp.
px: our usual standard pixel which maps to the screen pixel.
in: inches, with respect to the physical screen size.
mm: millimeters, with respect to the physical screen size.
pt: 1/72 of an inch, with respect to the physical screen size.

Formula for Conversion between Units

 px = dp * (dpi / 160)

dp to px in device

Following example may help understand better. The scaling occurs based on bucket size of 120(ldpi), 160(mdpi), 240(hdpi), 320(xhdpi), 480(xxhdpi) and 640(xxxhdpi). The Google suggested ratio for designing is 3:4:6:8:12 for ldpi:mdpi:hdpi:xhdpi:xxhdpi

A 150px X 150px image will occupy,

150 dp X 150 dp screen space in mdpi

100 dp X 100 dp screen space in hdpi

75 dp X 75 dp screen space in xhdpi

You may use the following DPI calculator to fix your image sizes and other dimensions when you wish to have an uniform UI design in all Android devices.

DPI Calculator in Java

/* Program output LDPI: 165.0 X 60.0 MDPI: 220.0 X 80.0 HDPI: 330.0 X 120.0 XHDPI: 440.0 X 160.0 XXHDPI: 660.0 X 240.0 XXXHDPI: 880.0 X 320.0 */   public class DPICalculator {  private final float LDPI = 120; private final float MDPI = 160; private final float HDPI = 240; private final float XHDPI = 320; private final float XXHDPI = 480; private final float XXXHDPI = 640;      private float forDeviceDensity; private float width; private float height;  public DPICalculator(float forDeviceDensity, float width, float height){     this.forDeviceDensity = forDeviceDensity;     this.width = width;     this.height = height; }  public static void main(String... args) {     DPICalculator dpiCalculator = new DPICalculator(240,330,120);     dpiCalculator.calculateDPI(); }   private float getPx(float dp, float value) {     float px = dp * (value / forDeviceDensity );             return px; }  private void calculateDPI() {      float ldpiW = getPx(LDPI,width);             float ldpiH =  getPx(LDPI,height);     float mdpiW = getPx(MDPI,width);             float mdpiH =  getPx(MDPI,height);             float hdpiW = getPx(HDPI,width);             float hdpiH =  getPx(HDPI,height);            float xdpiW = getPx(XHDPI,width);             float xdpiH =  getPx(XHDPI,height);     float xxdpiW = getPx(XXHDPI,width);             float xxdpiH =  getPx(XXHDPI,height);     float xxxdpiW = getPx(XXXHDPI,width);             float xxxdpiH =  getPx(XXXHDPI,height);      System.out.println("LDPI: " + ldpiW + " X " + ldpiH);     System.out.println("MDPI: " + mdpiW + " X " + mdpiH);     System.out.println("HDPI: " + hdpiW + " X " + hdpiH);     System.out.println("XHDPI: " + xdpiW + " X " + xdpiH);     System.out.println("XXHDPI: " + xxdpiW + " X " + xxdpiH);     System.out.println("XXXHDPI: " + xxxdpiW + " X " + xxxdpiH);            } }

More Information refer following link.

http://javapapers.com/android/difference-between-dp-dip-sp-px-in-mm-pt-in-android/

回答21:

Please read the answer from community wiki. Below mentioned are some information to be considered in addition to the above answers. Most Android developers miss this while developing apps, so I am adding these points.

sp = scale independent pixel

dp = density independent pixels

dpi = density pixels

I have gone through the above answers...not finding them exactly correct. sp for text size, dp for layout bounds - standard. But sp for text size will break the layout if used carelessly in most of the devices.

sp take the textsize of the device, whereas dp take that of device density standard( never change in a device) Say 100sp text can occupies 80% of screen or 100% of screen depending on the font size set in device

You can use sp for layout bounds also, it will work :) No standard app use sp for whole text

Use sp and dp for text size considering UX.

Dont use sp for text in toolbar( can use android dimens available for different screen sizes with dp)
Dont use sp for text in small bounded buttons, very smaller text, etc

Some people use huge FONT size in their phone for more readability, giving them small hardcoded sized text will be an UX issue. Put sp for text where necessary, but make sure it won't break the layout when user changes his settings.

Similarly if you have a single app supporting all dimensions, adding xxxhdpi assets increases the app size a lot. But now xxxhdpi phones are common so we have to include xxxhdpi assets atleast for icons in side bar, toolbar and bottom bar. Its better to move to vector images to have a uniform and better quality images for all screen sizes.

Also note that people use custom font in their phone. So lack of a font can cause problems regarding spacing and all. Say text size 12sp for a custom font may take some pixels extra than default font.

Refer google developer site for screendensities and basedensity details for android. https://developer.android.com/training/multiscreen/screendensities

回答22:

px - one pixel, same as to what is used in CSS, JavaScript, etc.
sp - scale-independent pixels
dip - density-independent pixels

Normally sp is used for font sizes, while dip is used (also called dp) for others.

回答23:

I've come across a good article about designing Android apps UI for different screen resolutions, and I'd like to leave it here just for somebody searching in this area. Yes, I know that it's somehow described in Google docs (and mentioned in the posts above), I read that but it was not good for me (yeah, I may be too stupid)). It remained unclear for me how to design layouts capable to handle different screen size. I hate DP concept and so on, when I need to implement a "flexible" UI layout for different screens. (Hey iOS developers - yes, you're right it's Storyboard concept).

Android has not bad UI concept, but lacks iOS Storyboard features, unfortunately. Designing flexible UI in Android is not easy thing (at the best).

Here goes the article that helped me to understand what to do in Android to make layouts for different screen sizes:

JMSTUDIO Blog :- Decide Android App Screen Size

How to Design UI for Android Apps for Different Screen Size

To design an app UI for different screen sizes, our initial design has to meet a minimum required space for each screen size. Android defines a minimum size (in dp) for each generalized screen type. Here is an Android screen size guideline. When we get the screen size in dp, it is not enough for us to design the Android app UI. For each screen size, we need to prepare graphics and bitmap images for each density. Here is an Android screen density guideline.

For easy calculation, we can follow the 3:4:6:8 scaling ratio between the four generalized densities. If we create a 36×36 pixel picture for ldpi device, the rest densities pictures size will be 48×48 for mdpi, 72×72 for hdpi, and 96×96 for xhdpi.

How to Design Android Apps UI in Photoshop

Many designers have problems for designing Android app UI in photoshop or other pixel based graphic design tools because of density-independent unit, dp. Designers don’t know how to map dp to pixel. Google also doesn’t give a clear Android UI design guide for them, though they give a basic formula for dp and pixel translation.

As Android’s definition, 1pd equal to 1px under 160 dpi device (mdpi). So we want to design an Android app for xlarge Android device with mdpi density, we can define our UI size in pixel as 960 pixel in width and 720px in height; Follow the same mapping rule, we can get following Android App screen size UI design guideline:

ADDED: If you interested in "flexible" UI too, have a look at this library: An Android SDK that provides a new size unit - sdp (scalable dp). This size unit scales with the screen size (this also mentioned in an answer here, about SDP library)

ADDED2 Google has finally understood usefulness of iOS Storeboard UI concept, and here goes ConstraintLayout for Android world: Build a Responsive UI with ConstraintLayout

回答24:

1) dp: (density independent pixels)

The number of pixels represented in one unit of dp will increase as the screen resolution increases (when you have more dots/pixels per inch). Conversely on devices with lower resolution, the number of pixels represented in on unit of dp will decrease. Since this is a relative unit, it needs to have a baseline to be compared with. This baseline is a 160 dpi screen. This is the equation: px = dp * (dpi / 160).

2) sp: (scale independent pixels)

This unit scales according to the screen dpi (similar to dp) as well as the user’s font size preference.

3) px: (pixels)

Actual pixels or dots on the screen.

For more details you can visit

Android Developer Guide > Dimension
Android Developer Guide > Screens

回答25:

Screen size in Android is grouped into categories ldpi, mdpi, hdpi, xhdpi, xxhdpi and xxxhdpi. Screen density is the amount of pixels within an area (like inch) of the screen. Generally it is measured in dots-per-inch (dpi).

PX(Pixels):

our usual standard pixel which maps to the screen pixel. px is meant for absolute pixels. This is used if you want to give in terms of absolute pixels for width or height. Not recommended.

DP/DIP(Density pixels / Density independent pixels):

dip == dp. In earlier Android versions dip was used and later changed to dp. This is alternative of px.
Generally we never use px because it is absolute value. If you use px to set width or height, and if that application is being downloaded into different screen sized devices, then that view will not stretch as per the screen original size.
dp is highly recommended to use in place of px. Use dp if you want to mention width and height to grow & shrink dynamically based on screen sizes.
if we give dp/dip, android will automatically calculate the pixel size on the basis of 160 pixel sized screen.

SP(Scale independent pixels):

scaled based on user’s font size preference. Fonts should use sp.
when mentioning the font sizes to fit for various screen sizes, use sp. This is similar to dp.Use sp especially for font sizes to grow & shrink dynamically based on screen sizes

Android Documentation says:

when specifying dimensions, always use either dp or sp units. A dp is a density-independent pixel that corresponds to the physical size of a pixel at 160 dpi. An sp is the same base unit, but is scaled by the user's preferred text size (it’s a scale-independent pixel), so you should use this measurement unit when defining text size

回答26:

The screen of a mobile phone is made up of thousands of tiny dots known as pixels (px). A pixel is the smallest element which goes to make the picture. The more the number of pixels to make a picture or wording, the sharper it becomes and makes the smartphone screen more easily readable.

Screen resolution is measured in terms of number of pixels on the screen. Screen resolution is a commonly-used specification when buying a device, but it's actually not that useful when designing for Android because thinking of screens in terms of pixels ignores the notion of physical size, which for a touch device is really really important.

Density independent pixel (dp or dip) allow the designer to create assets that appear in a expected way, no matter the resolution or density of target device.

A density independent pixel (dp or dip) is equal to one pixel at the baseline density or 160 dpi (dots per inch).

1 px/1dp = 160 dpi/160 dpi

2 px/1dp = 320 dpi(2x)/160 dpi

where,

dpi is dots per inch

So, at 320 dpi, 1 dp is equal to 2 px.

Formula

px/dp = dpi/160dpi

Dots per inch (dpi) is a measure of the sharpness (that is, the density of illuminated points) on a display screen. The dots per inch for a given picture resolution will differ based on the overall screen size since the same number of pixels are being spread out over a different space.

Working with density independent pixels help us to deal with a situation like where you have two devices with same pixel resolution, but differing amount of space. Suppose in a case, a tablet and phone has the same pixel resolution 1280 by 800 pixels (160 dpi) and 800 by 1280 pixels (320 dpi) respectively.

Now because a tablet is at baseline density (160 dpi) its physical and density independent pixels sizes are the same, 1280 by 800. The phone on the other hand has a higher pixel density, so it has half as many density independent pixels as physical pixels. So a phone has 400 by 640 density independent pixels. So using a density-independent pixel makes it easier to mentally picture that tablet has much more space than the phone.

Similarly, if you have two devices with similar screen size, but different pixel density, say one is 800 by 1280 pixels (320 dpi), and the other is 400 by 640 pixels (160 dpi), we don't need to define totally different layouts for these two devices as we can measure assets in terms of density independent pixel which is same for both devices.

800 by 1280 pixels (320dpi)=400 by 640 density independent pixel (dp)

400 by 640 pixels (160 dpi)=400 by 640 density independent pixel (dp)

Scale independent pixels(sp) is the preferred unit for font size. For accessibility purposes, Android allows users to customize their device's font size. Users that have trouble reading text can increase their device's font size. You can normally find this option in the display setting on your phone or tablet under font size. It's often also available through the accessibility settings.

With scale independent pixels, 16 sp is exactly the same as 16 dp when the device's font size is normal or 100%. But when device's font size is large, for example 125%, 16 sp will translate to 20 dp or 1.25 times 16.

If you use dp as the unit for font size, then that piece of text has a specific physical size no matter if the user has customize device's font size. Using sp units will make a better experience for people with impaired eyesight.

Reference: Udacity, Google

回答27:

sp: scale independent pixel

You should use it with texts because it is automatically scaled according to the font size that is being used by the user in his device.

px: pixel or picture element is the single point on the screen

回答28:

Pixel density

Screen pixel density and resolution vary depending on the platform. Device-independent pixels and scalable pixels are units that provide a flexible way to accommodate a design across platforms.

Calculating pixel density

The number of pixels that fit into an inch is referred to as pixel density. High-density screens have more pixels per inch than low-density ones....

The number of pixels that fit into an inch is referred to as pixel density. High-density screens have more pixels per inch than low-density ones. As a result, UI elements of the same pixel dimensions appear larger on low-density screens, and smaller on high-density screens.

To calculate screen density, you can use this equation:

Screen density = Screen width (or height) in pixels / Screen width (or height) in inches

Density independence

Screen pixel density and resolution vary depending on the platform. Device-independent pixels and scalable pixels are units that provide a flexible way to accommodate a design across platforms.

Calculating pixel density The number of pixels that fit into an inch is referred to as pixel density. High-density screens have more pixels per inch than low-density ones....

Density independence refers to the uniform display of UI elements on screens with different densities.

Density-independent pixels, written as dp (pronounced “dips”), are flexible units that scale to have uniform dimensions on any screen. Material UIs use density-independent pixels to display elements consistently on screens with different densities.

Low-density screen displayed with density independence
High-density screen displayed with density independence

Read full text https://material.io/design/layout/pixel-density.html

回答29:

I want to provide an easy way to understand dp. In fact, I think dp is the easiest one to understand. dp is just a physical length unit. It's of the same dimension as mm or inch. It's just convenient for us to write 50dp, 60dp rather than 50/160 inch or 60/160 inch, because one dp is just 1/160 inch whatever the screen size or resolution is.

The only problem is that, the android dpi of some screens are not accurate. For example, a screen classified to 160dpi may have 170dpi indeed. So the computation result of dp is fuzzy. It should be approximately the same as 1/160 inch.

回答30:

Before answering this question let me decrease the number of units first. So here you go: dp or dip are both the same and are known as Density-independent pixels.

1. px - stands for pixels. Pixels are a single dot, point on a screen. Generally in the mobile industry it is measured in ppi (pixels per inch). Screen resolution is directly proportional to ppi, the larger the number of pixels per inch the higher the screen resolution.

For example, if you draw an image of a size 200 px * 200 px, then its appearance must be different on a high-resolution device versus a low-resolution device. The reason is a 200 px image on a low-resolution phone will be look larger than on a high-resolution device.

Below images are showing a resolution of the same image on different phones -

Phone with High screen resolution
Phone with Low screen resolution

2. dip or dp - an abstract unit that is based on the physical density of the screen. These units are relative to a 160 dpi screen, so one dp is one pixel on a 160 dpi screen. The ratio of dp-to-pixel will change with the screen density, but not necessarily in direct proportion. "Density independence" refers to the uniform display of UI elements on screens with different densities.

Image which is showing 80px (left side image) and 80 dp (right-side image). Checkout difference.

A dp is equal to one physical pixel on a screen with a density of 160. To calculate dp:

dp = (width in pixels * 160) / screen density

3. sp - stands for scalable pixels. Generally sp is used for texts on the UI, and sp preserves the font settings. For example, if a user selected a larger font than 30 sp it will auto scale to appear large according to a user preference.

来源：https://stackoverflow.com/questions/2025282/what-is-the-difference-between-px-dip-dp-and-sp

标签

android

android-layout

user-interface

dimension

units-of-measurement

What is the difference between “px”, “dip”, “dp” and “sp”?

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