The Flex Basis property specifies the initial size of a flex item before extra space in the container is distributed. This is used in place of the width property in the flex layout.
Flex basis can accept values in Pixels (100px), Percentages (100%), ems (100em), rems (100rem) and keywords like auto. The default value is set to auto which sets the initial width based on the content of the item.
100px
100%
100em
100rem
auto
item-1 { flex-fasis: 400px; }
See the Pen Flex Basis by rehmaanali (@geekstrick) on CodePen.