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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 0 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

FeatureChromeEdgeFirefoxInternet ExplorerOperaSafari
Basic support

29

21 -webkit-

Yes

Yes -webkit-

201 2

49 -webkit-

44 -webkit- 3

18 — 284

115 6

10 -ms- 7 8

12.1

9

6.1 -webkit-

FeatureAndroid webviewChrome for AndroidEdge mobileFirefox for AndroidIE mobileOpera AndroidiOS Safari
Basic support

4.4

Yes -webkit-

?

Yes

Yes -webkit-

?1112.1

Yes

7.1 -webkit-

1. Since Firefox 28, multi-line flexbox is supported.

2. Before Firefox 32, Firefox wasn't able to animate values starting or stopping at 0.

3. 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.

4. 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.

5. Internet Explorer 11 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.

6. Internet Explorer 11 considers a unitless value in the flex-basis part to be syntactically invalid (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.

7. Internet Explorer 10 and 11 ignore 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.

8. Internet Explorer 10 and 11 consider a unitless value in the flex-basis part to be syntactically invalid (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