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The flex CSS property specifies how a flex item will grow or shrink so as to fit the space available in its flex container. This is a shorthand property that sets flex-grow, flex-shrink, and flex-basis.

/* Basic values */
flex: auto;
flex: initial;
flex: none;
flex: 2;

/* One value, unitless number: flex-grow */
flex: 2;

/* One value, width/height: flex-basis */
flex: 10em;
flex: 30px;

/* Two values: flex-grow | flex-basis */
flex: 1 30px;

/* Two values: flex-grow | flex-shrink */
flex: 2 2;

/* Three values: flex-grow | flex-shrink | flex-basis */
flex: 2 2 10%;

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

For most purposes, authors should set flex to one of the following values: auto, initial, none, or a positive unitless number. To see the effect of these values, try resizing the flex containers below:

auto
The item is sized according to its width and height properties, but grows to absorb any extra free space in the flex container, and shrinks to its minimum size to fit the container. This is equivalent to setting "flex: 1 1 auto".
initial
This is the default value. The item is sized according to its width and height properties. It shrinks to its minimum size to fit the container, but does not grow to absorb any extra free space in the flex container. This is equivalent to setting "flex: 0 1 auto".
none
The item is sized according to its width and height properties. It is fully inflexible: it neither shrinks nor grows in relation to the flex container. This is equivalent to setting "flex: 0 0 auto".
<positive-number>
The item is given the specified proportion of the free space in the container. This is equivalent to setting "flex: <positive-number> 1 0".

By default flex items don't shrink below their minimum content size. To change this, set the item's min-width or min-height.

Initial valueas each of the properties of the shorthand:
Applies toflex items, including in-flow pseudo-elements
Inheritedno
Mediavisual
Computed valueas each of the properties of the shorthand:
Animation typeas each of the properties of the shorthand:
Canonical orderorder of appearance in the formal grammar of the values

Syntax

The flex property may be specified using one, two, or three values.

One-value syntax: the value must be one of:

Two-value syntax: the first value must be a unitless <number> and it is interpreted as <flex-grow>. The second value must be one of:

Three-value syntax:

Values

<'flex-grow'>
Defines the flex-grow of the flex item. See <number> for more details. Negative values are considered invalid. Defaults to 0 when omitted.
<'flex-shrink'>
Defines the flex-shrink of the flex item. See <number> for more details. Negative values are considered invalid. Defaults to 1 when omitted.
<'flex-basis'>
Defines the flex-basis of the flex item. Any value valid for width and height properties are accepted. A preferred size of 0 must have a unit to avoid being interpreted as a flexibility. Defaults to auto when omitted.
none
This keyword computes to 0 0 auto.

When using one or two unitless values, flex-basis changes from auto to 0. More information can be found in the draft for the Flexible Box Layout Module.

Formal syntax

none | [ <'flex-grow'> <'flex-shrink'>? || <'flex-basis'> ]

Example

#flex-container {
  display: flex;
  flex-direction: row;
}

#flex-container > .flex-item {
  flex: auto;
}

#flex-container > .raw-item {
  width: 5rem;
}
<div id="flex-container">
  <div class="flex-item" id="flex">Flex box (click to toggle raw box)</div>
  <div class="raw-item" id="raw">Raw box</div>
</div>

Result

Specifications

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

Browser compatibility

Feature Firefox (Gecko) Chrome Edge Internet Explorer Opera Safari
Basic support 18.0 (18.0)[1]
20.0 (20.0)
28.0 (28.0)[2]
21.0-webkit
29.0
(Yes)-webkit
(Yes)
10.0-ms[3]
11.0[3]
12.10

6.1-webkit
9.0

Feature Firefox Mobile (Gecko) Android Edge IE Phone Opera Mobile Safari Mobile
Basic support ? 4.4 (Yes)-webkit
(Yes)
11 12.10 7.1-webkit

[1] In Gecko 18.0 (Firefox 18.0 / Thunderbird 18.0 / SeaMonkey 2.15) and 19.0 (Firefox 19.0 / Thunderbird 19.0 / SeaMonkey 2.16) flexbox support is hidden behind the about:config preference layout.css.flexbox.enabled, defaulting to false.

[2] Multi-line flexbox is supported since Gecko 28.0 (Firefox 28.0 / Thunderbird 28.0 / SeaMonkey 2.25).

In addition to the unprefixed support, Gecko 48.0 (Firefox 48.0 / Thunderbird 48.0 / SeaMonkey 2.45) added support for a -webkit prefixed version of the property for web compatibility reasons behind the preference layout.css.prefixes.webkit, defaulting to false. Since Gecko 49.0 (Firefox 49.0 / Thunderbird 49.0 / SeaMonkey 2.46) the preference defaults to true.

[3] Internet Explorer 10-11 (but not 12+) ignores uses of calc() in the flex-basis part of the flex syntax. This can be worked around by using the longhand properties instead of the shorthand. See Flexbug #8 for more info. Furthermore a flex declaration with a unitless value in its flex-basis part is considered syntactically invalid in those versions and will thus be ignored. A workaround is to always include a unit in the flex-basis part of the flex shorthand value. See Flexbug #4 for more info.

See also