Using margin on flex items

I was under the impression that a margin can be added to flex items/children, and flexbox should automatically take that into account and calculate the correct spacing between the items.

I can't seem to get this working as I would like though.

Fiddle here: https://jsfiddle.net/dba5ehcw/1/

.flex-item{    border: 1px solid blue;    box-sizing: border-box;    height: 160px;    width: 50%;}

So each flex item at the moment is half the width of the container, and they flow nicely next to each other.

I would like to be able to add a margin of say, 1em to the flex-items in order to give them some breathing room, but in doing so, they become larger than the 50% and no longer stack next to each other on the same line because they are too wide.

Is there a way to use margin on the flex-items and have the flexbox container take this into account and adjust (decrease) their widths accordingly?