Flex Basis

The flex-basis property defines the size of the flex-item along the main axis of the flex container. The main axis is horizontal if flex-direction is set to row and it’ll be vertical if the flex-direction property is set to column.


flex-basis: auto | content | <width> | <height>;

flex-basis: auto

flex-basis: auto looks up the main size of the element and defines the size. For example, on a horizontal flex container, auto will look for width and height if the container axis is vertical.

If no size is specified, auto will fall back to content.

flex-basis: content

flex-basis: content resolves the size based on the element’s content, unless width or height is set through normal box-sizing.

In both the cases where flex-basis is either auto or content, if main size is specified, that size will take priority.


This is just as specifying width or height, but only more flexible. flex-basis: 20em; will set the initial size of the element to 20em. Its final size will be based on available space, flex-grow multiple and flex-shrink multiple.

The specification suggests use of flex shorthand property. This helps write flex-basis along with flex-grow and flex-shrink properties.


Here is rows of individual flex containers and individual flex elements showing how flex-basis affects the box-sizing.

effect of flex-basis on horizontal axis

When the flex-direction is column, the same flex-basis will control the height property. You can see it in the example below:

example of flex-basis controlling height in a vertical container

More Information:

You can fund additional references about the flex basis property on the following pages:

More info on Flexbox: