flex-basis
Baseline Widely available
This feature is well established and works across many devices and browser versions. It’s been available across browsers since September 2015.
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: It is recommended to use the flex
shorthand instead of separate flex-grow
, flex-shrink
, and flex-basis
declarations. We have separated them here as this document is about one of the shorthand components: the flex-basis
property.
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 changes the flex-basis
value set on the first flex item, causing it to grow or shrink to fill the available space. The other flex items will also change size; they will be at least min-content
-sized. For example, when the flex-basis
of the first item is set to 200px
, it will start at 200px
but then shrink to fit the space available.
If flex-basis
is set to a value other than auto
and there is a width
(or height
in case of flex-direction: column
) set for that same flex item, the flex-basis
value takes precedence.
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'>
-
Any of the following units:
<length>
sets an absolute value.<percentage>
sets a percentage of the width or height of the containing block's content area. Percentage values offlex-basis
are resolved against the flex container. If the flex container's size is indefinite, the used value forflex-basis
iscontent
.auto
uses the value of thewidth
in horizontal writing mode, and the value of theheight
in vertical writing mode; when the corresponding value is alsoauto
, thecontent
value is used instead.max-content
sets the intrinsic preferred width.min-content
sets the intrinsic minimum width.fit-content
sets the maximum possible size of a containing block's content area, bounded by themin-content
andmax-content
values, and calculated based on the content of the current element.
content
-
Indicates automatic sizing, based on the flex item's content.
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'>
<width> =
auto |
<length-percentage [0,∞]> |
min-content |
max-content |
fit-content( <length-percentage [0,∞]> ) |
<calc-size()> |
<anchor-size()>
<length-percentage> =
<length> |
<percentage>
<calc-size()> =
calc-size( <calc-size-basis> , <calc-sum> )
<anchor-size()> =
anchor-size( [ <anchor-element> || <anchor-size> ]? , <length-percentage>? )
<calc-size-basis> =
<intrinsic-size-keyword> |
<calc-size()> |
any |
<calc-sum>
<calc-sum> =
<calc-product> [ [ '+' | '-' ] <calc-product> ]*
<anchor-element> =
<dashed-ident>
<anchor-size> =
width |
height |
block |
inline |
self-block |
self-inline
<calc-product> =
<calc-value> [ [ '*' | '/' ] <calc-value> ]*
<calc-value> =
<number> |
<dimension> |
<percentage> |
<calc-keyword> |
( <calc-sum> )
<calc-keyword> =
e |
pi |
infinity |
-infinity |
NaN
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
Flex basis 0
vs 0%
This example demonstrates the difference between a flex-basis
of 0
versus a flex-basis
of 0%
when flex-direction
is set to column
and the flex containers and flex items don't have a set height; while 0
is an absolute length, percentage flex-basis values resolve to content
values.
HTML
We include two same-structure flex containers. These which will be styled similarly, except for their flex-basis
values. The containers each have two children: a heading <div>
and a <section>
. The <section>
element has a content <div>
child, which will not be set as a flex item but will be given a height.
<div class="container basis-0">
<div>heading</div>
<section>
flex-basis: 0;
<div class="content"></div>
</section>
</div>
<div class="container basis-0-percent">
<div>heading</div>
<section>
flex-basis: 0%;
<div class="content"></div>
</section>
</div>
CSS
We style the containers as inline flex containers that will appear side by side to better enable comparing them. We set the flex-direction
to the column. The first container's flex items have a flex-basis
value of 0
and the second container's flex item have a flex-basis
value of 0%
. Neither the flex containers nor their flex items have a height explicitly set, but the heights of section
elements can not exceed 200px
and their children have height 300px
.
.container {
width: 40vw;
padding: 1rem;
border: 1px dashed blue;
display: inline-flex;
flex-direction: column;
}
section {
border: 1px solid red;
overflow: auto;
min-height: 200px;
}
.content {
background: wheat;
height: 300px;
}
.container.basis-0 > * {
flex-basis: 0;
}
.container.basis-0-percent > * {
flex-basis: 0%;
}
Results
In the first container, with flex-basis: 0
, the <section>
element has an initial main size of zero, and it grows to the 200px
height limit. In the second container, with flex-basis: 0%
, the <section>
element has an initial main size of 300px
because, as the flex container doesn't have a set height, the percentage flex-basis values resolve to the content
value.
Specifications
Specification |
---|
CSS Flexible Box Layout Module Level 1 # flex-basis-property |
Browser compatibility
BCD tables only load in the browser