How to get the scale percentage of an image when using background-size:cover?

Question

I need to figure out how much an image has scaled.

I am setting a background image to the body with the background-size: cover.

Basic

Answer 1

If we suppose your image has a dimension of WxH and the screen size is AxB then it should be the biggest value between A/W and B/H.

Some examples:

.box {
  width:400px;
  height:200px;
  background:url(https://picsum.photos/300/300?image=0) center/cover;
}
/*
we will have 1.333 = 400/300 and 0.6667 = 200/300
*/
img {
 transform:scale(1.3333);
 transform-origin:top left;
}

<div class="box">

</div>
<img src="https://picsum.photos/300/300?image=0">

.box {
  width:300px;
  height:200px;
  background:url(https://picsum.photos/500/500?image=0) center/cover;
}
/*
we will have 0.6 = 300/500 and 0.4 = 200/500
*/
img {
 transform:scale(0.6);
 transform-origin:top left;
}

<div class="box">

</div>
<img src="https://picsum.photos/500/500?image=0">

.box {
  width:100px;
  height:300px;
  background:url(https://picsum.photos/400/300?image=0) center/cover;
  display:inline-block;
}
/*
we will have 0.25 = 100/400 and 1 = 300/300
*/
img {
 transform:scale(1);
 transform-origin:top left;
}

<div class="box">

</div>
<img src="https://picsum.photos/400/300?image=0">

With contain we take the smallest value:

.box {
  width:400px;
  height:200px;
  background:url(https://picsum.photos/300/300?image=0) left/contain no-repeat;
}
/*
we will have 1.333 = 400/300 and 0.6667 = 200/300
*/
img {
 transform:scale(0.6667);
 transform-origin:top left;
}

<div class="box">

</div>
<img src="https://picsum.photos/300/300?image=0">

.box {
  width:300px;
  height:200px;
  background:url(https://picsum.photos/500/500?image=0) left/contain no-repeat;
}
/*
we will have 0.6 = 300/500 and 0.4 = 200/500
*/
img {
 transform:scale(0.4);
 transform-origin:top left;
}

<div class="box">

</div>
<img src="https://picsum.photos/500/500?image=0">

.box {
  width:100px;
  height:300px;
  background:url(https://picsum.photos/400/300?image=0) top/contain no-repeat;
  display:inline-block;
}
/*
we will have 0.25 = 100/400 and 1 = 300/300
*/
img {
 transform:scale(0.25);
 transform-origin:top left;
}

<div class="box">

</div>
<img src="https://picsum.photos/400/300?image=0">