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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.

Try it

In this example, the flex-grow and flex-shrink properties are both set to 1 on all three items, indicating that the flex item can grow and shrink from the initial flex-basis.

The demo then changes the flex-basis on the first item. It will then grow and shrink from that flex-basis. This means that, for example, when the flex-basis of the first item is 200px, it will start out at 200px but then shrink to fit the space available with the other items being at least min-content sized.

The image below shows how the Firefox Flexbox Inspector helps you understand the size items become:

The Firefox Flexbox Inspector showing the size of the item once it has shrunk.

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: 50%;
flex-basis: auto;

/* Intrinsic sizing keywords */
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: revert;
flex-basis: revert-layer;
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.

  • 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

flex-basis = 
  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';
}

Results

Specifications

Specification
CSS Flexible Box Layout Module Level 1
# flex-basis-property

Browser compatibility

Desktop Mobile
Chrome Edge Firefox Internet Explorer Opera Safari WebView Android Chrome Android Firefox for Android Opera Android Safari on IOS Samsung Internet
flex-basis
29
22
12
12
49
22
Since Firefox 28, multi-line flexbox is supported.
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.
15
12.1
9
7
≤37
4.4
29
25
49
22
Since Firefox 28, multi-line flexbox is supported.
14
12.1
9
7
2.0
1.5
auto
22
12
22
11
12.1
9
7
4.4
25
22
12.1
9
7
1.5
content
94
94
12-79
61
No
80
preview
94
94
61
No
No
17.0
fit-content
94
94
94
22
No
80
16
94
94
94
22
No
16
17.0
max-content
94
94
66
22
No
80
16
94
94
66
22
No
16
17.0
min-content
94
94
66
22
No
80
16
94
94
66
22
No
16
17.0

See also

© 2005–2022 MDN contributors.
Licensed under the Creative Commons Attribution-ShareAlike License v2.5 or later.
https://developer.mozilla.org/en-US/docs/Web/CSS/flex-basis