The flex-basis property is a sub-property of the Flexible Box Layout module.

It specifies the initial size of the flex item, before any available space is distributed according to the flex factors. When omitted from the flex shorthand, its specified value is the length zero.

A flex-basis value set to auto sizes the element according to its size property (which can itself be the keyword auto, which sizes the element based on its contents).


flex-basis: <width>

.flex-item {
  flex-basis: 100px;

Please note that as for any width, negative lengths are invalid.


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Related Properties

Other Resources

Browser Support

  • (modern) means the recent syntax from the specification (e.g. display: flex;)
  • (hybrid) means an odd unofficial syntax from 2011 (e.g. display: flexbox;)
  • (old) means the old syntax from 2009 (e.g. display: box;)
Chrome Safari Firefox Opera IE Android iOS
21+ (modern)
20- (old)
3.1+ (old) 2-21 (old)
22+ (new)
12.1+ (modern) 10+ (hybrid) 2.1+ (old) 3.2+ (old)

Blackberry browser 10+ supports the new syntax.

For more informations about how to mix syntaxes in order to get the best browser support, please refer to this article (CSS-Tricks) or this article (DevOpera).


  1. User Avatar

    The article should maybe be updated to say that the spec recommends using the ‘flex’ shorthand over the ‘flex-basis’ longhand.

  2. User Avatar
    Permalink to comment#

    codepen demo doesn’t show flex-basis

    • User Avatar
      Permalink to comment#

      It uses the flex shorthand rule, the last value is for flex-basis and the demo has an explicit value set in em’s.

  3. User Avatar
    Permalink to comment#

    Can anyone help me to solve this problem?

  4. User Avatar
    Permalink to comment#

    What exactly is the size property?

  5. User Avatar
    Permalink to comment#

    Hi there. Thanks for the article and this website in general. May I ask if this post is still usable now since it’s a bit old? Thanks.

    • User Avatar
      Geoff Graham
      Permalink to comment#

      Yep, everything here is relevant. You might also consider referring to the full flexbox guide for a complete overview of all the available flex properties with examples.

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