CSS flex-basis

The CSS flex-basis property sets the flex basis on a flex item. The property specifies the initial main size of the flex item, before free space is distributed according to the flex factors.

This property takes the same values as the width and height properties (except auto is treated differently), as well as an additional content keyword.

The W3C encourages authors to use this flex shorthand property rather than its longhand properties directly, as the shorthand correctly resets any unspecified components to accommodate common uses.


Possible Values

The flex basis is the computed value of the flex item's main size property. If that value is itself auto, then the flex basis is automatically-determined based on its content (i.e. sized as for content).
Specifies automatic sizing, based on the flex item's content.
For all other values, flex-basis is resolved the same way as with the width property.

In addition, all CSS properties also accept the following CSS-wide keyword values as the sole component of their property value:

Represents the value specified as the property's initial value.
Represents the computed value of the property on the element's parent.
This value acts as either inherit or initial, depending on whether the property is inherited or not. In other words, it sets all properties to their parent value if they are inheritable or to their initial value if not inheritable.

Basic Property Information

Initial Value
Applies To
Flex items
Computed Value
As specified, with lengths made absolute
Yes (see example)

Example Code

Basic CSS

Working Example within an HTML Document

Try it

CSS Specifications

About Flexbox

Flexbox refers to the Flexible Box Layout module introduced in CSS3. A flex container is an element with either display: flex or display: inline-flex.

In the flex layout model, the children of a flex container can be laid out in any direction, and can "flex" their sizes, either growing to fill unused space or shrinking to avoid overflowing the parent.

For more information on flex items, see the flex property.

Browser Support

The following table provided by Caniuse.com shows the level of browser support for this feature.

Vendor Prefixes

For maximum browser compatibility many web developers add browser-specific properties by using extensions such as -webkit- for Safari, Google Chrome, and Opera (newer versions), -ms- for Internet Explorer, -moz- for Firefox, -o- for older versions of Opera etc. As with any CSS property, if a browser doesn't support a proprietary extension, it will simply ignore it.

This practice is not recommended by the W3C, however in many cases, the only way you can test a property is to include the CSS extension that is compatible with your browser.

The major browser manufacturers generally strive to adhere to the W3C specifications, and when they support a non-prefixed property, they typically remove the prefixed version. Also, W3C advises vendors to remove their prefixes for properties that reach Candidate Recommendation status.

Many developers use Autoprefixer, which is a postprocessor for CSS. Autoprefixer automatically adds vendor prefixes to your CSS so that you don't need to. It also removes old, unnecessary prefixes from your CSS.

You can also use Autoprefixer with preprocessors such as Less and Sass.