W3cubDocs

/CSS

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 value auto
Applies to flex items, including in-flow pseudo-elements
Inherited no
Percentages refer to the flex container's inner main size
Computed value as specified, but with relative lengths converted into absolute lengths
Animation type a 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 compatibilityUpdate compatibility data on GitHub

Desktop
Chrome Edge Firefox Internet Explorer Opera Safari
flex-basis 29
29
22
Prefixed
Prefixed Implemented with the vendor prefix: -webkit-
12
12
12
Prefixed
Prefixed Implemented with the vendor prefix: -webkit-
22
22
Since Firefox 28, multi-line flexbox is supported.
49
Prefixed
Prefixed Implemented with the vendor prefix: -webkit-
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.
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.
11
11
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.
12.1
12.1
15
Prefixed
Prefixed Implemented with the vendor prefix: -webkit-
9
9
7
Prefixed
Prefixed Implemented with the vendor prefix: -webkit-
auto 22 12 18 11 12.1 9
9
7
Prefixed
Prefixed Implemented with the vendor prefix: -webkit-
content No 12 — 79 61 No No No
max-content No No 66
66
22
Prefixed
Prefixed Implemented with the vendor prefix: -moz-
No No No
min-content No No 66
66
22
Prefixed
Prefixed Implemented with the vendor prefix: -moz-
No No No
Mobile
Android webview Chrome for Android Firefox for Android Opera for Android Safari on iOS Samsung Internet
flex-basis 4.4
4.4
≤37
Prefixed
Prefixed Implemented with the vendor prefix: -webkit-
29
29
25
Prefixed
Prefixed Implemented with the vendor prefix: -webkit-
22
22
Since Firefox 28, multi-line flexbox is supported.
49
Prefixed
Prefixed Implemented with the vendor prefix: -webkit-
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.
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.
12.1
12.1
14
Prefixed
Prefixed Implemented with the vendor prefix: -webkit-
9
9
7
Prefixed
Prefixed Implemented with the vendor prefix: -webkit-
2.0
2.0
1.5
Prefixed
Prefixed Implemented with the vendor prefix: -webkit-
auto ≤37 25 18 12.1 9
9
7
Prefixed
Prefixed Implemented with the vendor prefix: -webkit-
1.5
content No No 61 No No No
max-content No No 66
66
22
Prefixed
Prefixed Implemented with the vendor prefix: -moz-
No No No
min-content No No 66
66
22
Prefixed
Prefixed Implemented with the vendor prefix: -moz-
No No No

See also

© 2005–2020 Mozilla and individual contributors.
Licensed under the Creative Commons Attribution-ShareAlike License v2.5 or later.
https://developer.mozilla.org/en-US/docs/Web/CSS/flex-basis