The flex-basis CSS property sets the initial main size of a flex item. It sets the size of the content box unless otherwise set with box-sizing.

Note: in case both flex-basis (other than auto) and width (or height in case of flex-direction: column) are set for an element, flex-basis has priority.

Syntax

/* Specify <'width'> */
flex-basis: 10em;      
flex-basis: 3px;
flex-basis: auto;

/* Intrinsic sizing keywords */
flex-basis: fill;
flex-basis: max-content;
flex-basis: min-content;
flex-basis: fit-content;

/* Automatically size based on the flex item’s content */
flex-basis: content;

/* Global values */
flex-basis: inherit;
flex-basis: initial;
flex-basis: unset;

The flex-basis property is specified as either the keyword content or a <'width'>.

Values

<'width'>
An absolute <length>, a <percentage> of the parent flex container's main size property, or the keyword auto. Negative values are invalid.
content
Indicates automatic sizing, based on the flex item’s content.
Note: This value was not present in the initial release of Flexible Box Layout, and thus some older implementations will not support it. The equivalent effect can be had by using auto together with a main size (width or height) of auto.

History:

  • Originally, flex-basis:auto meant "look at my width or height property".
  • Then, flex-basis:auto was changed to mean automatic sizing, and "main-size" was introduced as the "look at my width or height property" keyword. It was implemented in bug 1032922.
  • Then, that change was reverted in bug 1093316, so auto once again means "look at my width or height property"; and a new content keyword is being introduced to trigger automatic sizing. (bug 1105111 covers adding that keyword).

Formal syntax

content | <'width'>

Example

HTML

<ul class="container">
  <li class="flex flex1">1: flex-basis test</li>
  <li class="flex flex2">2: flex-basis test</li>
  <li class="flex flex3">3: flex-basis test</li>
  <li class="flex flex4">4: flex-basis test</li>
  <li class="flex flex5">5: flex-basis test</li>
</ul>

<ul class="container">
  <li class="flex flex6">6: flex-basis test</li>
</ul>

CSS

.container {
  font-family: arial, sans-serif;
  margin: 0;
  padding: 0;
  list-style-type: none;
  display: flex;
  flex-wrap: wrap;
}

.flex {
  background: #6AB6D8;
  padding: 10px;
  margin-bottom: 50px;
  border: 3px solid #2E86BB;
  color: white;
  font-size: 20px;
  text-align: center;
  position: relative;
}

.flex:after {
  position: absolute;
  z-index: 1;
  left: 0;
  top: 100%;
  margin-top: 10px;
  width: 100%;
  color: #333;
  font-size: 18px;
}

.flex1 {
  flex-basis: auto;
}

.flex1:after {
  content: 'auto';
}

.flex2 {
  flex-basis: max-content;
}

.flex2:after {
  content: 'max-content';
}

.flex3 {
  flex-basis: min-content;
}

.flex3:after {
  content: 'min-content';
}

.flex4 {
  flex-basis: fit-content;
}

.flex4:after {
  content: 'fit-content';
}

.flex5 {
   flex-basis: content;
}

.flex5:after {
  content: 'content';
}

.flex6 {
  flex-basis: fill;
}

.flex6:after {
  content: 'fill';
}

Results

Specifications

Specification Status Comment
CSS Flexible Box Layout Module
The definition of 'flex-basis' in that specification.
Candidate Recommendation Initial definition

Initial valueauto
Applies toflex items, including in-flow pseudo-elements
Inheritedno
Percentagesrefer to the flex container's inner main size
Mediavisual
Computed valueas specified, but with relative lengths converted into absolute lengths
Animation typea length, percentage or calc();
Canonical orderthe length or percentage before the keyword, if both are present

Browser compatibility

Update compatibility data on GitHub
DesktopMobile
ChromeEdgeFirefoxInternet ExplorerOperaSafariAndroid webviewChrome for AndroidEdge MobileFirefox for AndroidOpera for AndroidiOS SafariSamsung Internet
Basic supportChrome Full support 29
Full support 29
Full support 21
Prefixed
Prefixed Requires the vendor prefix: -webkit-
Edge Full support 12
Full support 12
Full support 12
Prefixed
Prefixed Requires the vendor prefix: -webkit-
Firefox Full support 22
Notes
Full support 22
Notes
Notes Since Firefox 28, multi-line flexbox is supported.
Full support 49
Prefixed
Prefixed Requires the vendor prefix: -webkit-
Full support 44
Prefixed Disabled
Prefixed Requires the vendor prefix: -webkit-
Disabled From version 44: this feature is behind the layout.css.prefixes.webkit preference (needs to be set to true). To change preferences in Firefox, visit about:config.
No support 18 — 28
Disabled
Disabled From version 18 until version 28 (exclusive): this feature is behind the layout.css.flexbox.enabled preference (needs to be set to true). To change preferences in Firefox, visit about:config.
IE Full support 11
Notes
Full support 11
Notes
Notes When a non-auto flex-basis is specified, Internet Explorer 10 and 11 always uses a content-box box model to calculate the size of a flex item, even if box-sizing: border-box is applied to the element. See Flexbug #7 for more info.
Opera Full support 12.1
Full support 12.1
Full support 15
Prefixed
Prefixed Requires the vendor prefix: -webkit-
Safari Full support 9
Full support 9
Full support 7
Prefixed
Prefixed Requires the vendor prefix: -webkit-
WebView Android Full support 4.4
Full support 4.4
Full support Yes
Prefixed
Prefixed Requires the vendor prefix: -webkit-
Chrome Android Full support 29
Full support 29
Full support 25
Prefixed
Prefixed Requires the vendor prefix: -webkit-
Edge Mobile Full support Yes
Full support Yes
Full support Yes
Prefixed
Prefixed Requires the vendor prefix: -webkit-
Firefox Android Full support 22
Notes
Full support 22
Notes
Notes Since Firefox 28, multi-line flexbox is supported.
Full support 49
Prefixed
Prefixed Requires the vendor prefix: -webkit-
Full support 44
Prefixed Disabled
Prefixed Requires the vendor prefix: -webkit-
Disabled From version 44: this feature is behind the layout.css.prefixes.webkit preference (needs to be set to true). To change preferences in Firefox, visit about:config.
No support 18 — 28
Disabled
Disabled From version 18 until version 28 (exclusive): this feature is behind the layout.css.flexbox.enabled preference (needs to be set to true). To change preferences in Firefox, visit about:config.
Opera Android Full support 12.1
Full support 12.1
Full support 15
Prefixed
Prefixed Requires the vendor prefix: -webkit-
Safari iOS Full support 9.2Samsung Internet Android ?
autoChrome Full support 21Edge Full support 12Firefox Full support 18IE Full support 11Opera Full support 12.1Safari Full support 7
Prefixed
Full support 7
Prefixed
Prefixed Requires the vendor prefix: -webkit-
WebView Android ? Chrome Android ? Edge Mobile Full support YesFirefox Android Full support 18Opera Android Full support 12.1Safari iOS ? Samsung Internet Android ?
contentChrome No support NoEdge Full support 12Firefox Full support 61IE No support NoOpera No support NoSafari No support NoWebView Android ? Chrome Android No support NoEdge Mobile Full support YesFirefox Android Full support 61Opera Android No support NoSafari iOS No support NoSamsung Internet Android No support No

Legend

Full support  
Full support
No support  
No support
Compatibility unknown  
Compatibility unknown
See implementation notes.
See implementation notes.
User must explicitly enable this feature.
User must explicitly enable this feature.
Requires a vendor prefix or different name for use.
Requires a vendor prefix or different name for use.

See also