5 items per row, auto-resize items in flexbox

Answer 1

You are right in giving a flex-basis: 20% but you have to adjust for the 4px margin on each flex item for it to wrap properly.

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Equal Width Flex items in the last row

Use flex: 0 1 calc(20% - 8px) - this means the item won't grow beyond 20% of width (adjusting for margin) and can shrink based on the container width. See demo below:

.container {
  background: gray;
  width: 600px;
  height: 200px; /* height given for illustration */
  display: flex;
  flex-flow: row wrap;
  position: relative;
}

.item {
  background: blue;
  margin: 4px;
  flex: 0 1 calc(20% - 8px); /* <-- adjusting for margin */
}

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Another approach is a bit hacky - you can keep flex-grow set to one and flex-basis: calc(20% - 4px) using flex: 1 1 calc(20% - 4px), and use a pseudo element that fills the remaining space:

.container {
  background: gray;
  width: 600px;
  height: 200px; /* height given for illustration */
  display: flex;
  flex-flow: row wrap;
  position: relative;
}

.item {
  background: blue;
  margin: 4px;
  flex: 1 1 calc(20% - 8px); /* <-- adjusting for margin */
}

.container:after {
  content: '';
  display: block;
  flex: 999; /* grow by a large number */
}

If you don't have the n item per row requirement then you can refer this:

• Unordered list that acts like grid-auto-flow dense

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Flex items in last row expands to fill the available space

If in a row you have less than 5 items and you want them to fill in the remaining space use flex: 1 1 calc(20% - 8px) (note that flex-grow is set to 1 here so that the flex items in the last rows expand to fill the remaining space):

.container {
  background: gray;
  width: 600px;
  height: 200px;  /* height given for illustration */
  display: flex;
  flex-flow: row wrap;
  position: relative;
}

.item {
  background: blue;
  margin: 4px;
  flex: 1 1 calc(20% - 8px);  /* <-- adjusting for margin */
}