flex-basis

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. Defaults to auto.
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 definition

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

Formal syntax

content | <'width'>

Examples

Setting flex item initial sizes

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: 14px;
  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: 12px;
}

.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

Browser compatibility

DesktopMobile
ChromeEdgeFirefoxInternet ExplorerOperaSafariAndroid webviewChrome for AndroidFirefox for AndroidOpera for AndroidSafari on iOSSamsung Internet
flex-basisChrome Full support 29
Full support 29
Full support 22
Prefixed
Prefixed Implemented with the vendor prefix: -webkit-
Edge Full support 12
Full support 12
Full support 12
Prefixed
Prefixed Implemented with 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 Implemented with the vendor prefix: -webkit-
Full support 44
Prefixed Disabled
Prefixed Implemented with 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 Implemented with the vendor prefix: -webkit-
Safari Full support 9
Full support 9
Full support 7
Prefixed
Prefixed Implemented with the vendor prefix: -webkit-
WebView Android Full support 4.4
Full support 4.4
Full support ≀37
Prefixed
Prefixed Implemented with the vendor prefix: -webkit-
Chrome Android Full support 29
Full support 29
Full support 25
Prefixed
Prefixed Implemented with 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 Implemented with the vendor prefix: -webkit-
Full support 44
Prefixed Disabled
Prefixed Implemented with 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 14
Prefixed
Prefixed Implemented with the vendor prefix: -webkit-
Safari iOS Full support 9
Full support 9
Full support 7
Prefixed
Prefixed Implemented with the vendor prefix: -webkit-
Samsung Internet Android Full support 2.0
Full support 2.0
Full support 1.5
Prefixed
Prefixed Implemented with the vendor prefix: -webkit-
autoChrome Full support 22Edge Full support 12Firefox Full support 18IE Full support 11Opera Full support 12.1Safari Full support 7
Prefixed
Full support 7
Prefixed
Prefixed Implemented with the vendor prefix: -webkit-
WebView Android Full support ≀37Chrome Android Full support 25Firefox Android Full support 18Opera Android Full support 12.1Safari iOS Full support 7
Prefixed
Full support 7
Prefixed
Prefixed Implemented with the vendor prefix: -webkit-
Samsung Internet Android Full support 1.5
contentChrome No support NoEdge No support 12 — 79Firefox Full support 61IE No support NoOpera No support NoSafari No support NoWebView Android No support NoChrome Android No support NoFirefox Android Full support 61Opera Android No support NoSafari iOS No support NoSamsung Internet Android No support No
max-contentChrome No support NoEdge No support NoFirefox Full support 66
Full support 66
Full support 22
Prefixed
Prefixed Implemented with the vendor prefix: -moz-
IE No support NoOpera No support NoSafari No support NoWebView Android No support NoChrome Android No support NoFirefox Android Full support 66
Full support 66
Full support 22
Prefixed
Prefixed Implemented with the vendor prefix: -moz-
Opera Android No support NoSafari iOS No support NoSamsung Internet Android No support No
min-contentChrome No support NoEdge No support NoFirefox Full support 66
Full support 66
Full support 22
Prefixed
Prefixed Implemented with the vendor prefix: -moz-
IE No support NoOpera No support NoSafari No support NoWebView Android No support NoChrome Android No support NoFirefox Android Full support 66
Full support 66
Full support 22
Prefixed
Prefixed Implemented with the vendor prefix: -moz-
Opera Android No support NoSafari iOS No support NoSamsung Internet Android No support No

Legend

Full support
Full support
No support
No support
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