How to make flexbox items the same size?

You need to add width: 0 to make columns equal if contents of the items make it grow bigger.

.item {
  flex: 1 1 0;
  width: 0;
}

Detail: flex: 1 1 0 is the same as flex-grow: 1; flex-shrink: 1; flex-basis: 0; and if the parent container can not provide enough space for the native-size added together of every item (no space to grow), we need to make the width: 0 to give every item the same start point to grow.

Set them so that their flex-basis is 0 (so all elements have the same starting point), and allow them to grow:

flex: 1 1 0px

Your IDE or linter might mention that the unit of measure 'px' is redundant. If you leave it out (like: flex: 1 1 0), IE will not render this correctly. So the px is required to support Internet Explorer, as mentioned in the comments by @fabb;