The flex CSS property is a shorthand property specifying the ability of a flex item to alter its dimensions to fill available space. Flex items can be stretched to use available space proportional to their flex grow factor or their flex shrink factor to prevent overflow.

  • Initial value the concatenation of the initial values of its longhand properties:
    • flex-grow: 0
    • flex-shrink: 1
    • flex-basis: auto
  • Applies to flex items, including in-flow pseudo-elements
  • Inherited no
  • Media visual
  • Computed value as each of the properties of the shorthand:
    • flex-grow: as specified
    • flex-shrink: as specified
    • flex-basis: as specified, with lengths made absolute
  • Animatable as each of the properties of the shorthand:
    • flex-grow: yes, as a number
    • flex-shrink: yes, as a number
    • flex-basis: yes, as a length, percentage or calc(); when both values are lengths, they are interpolated as lengths; when both values are percentages, they are interpolated as percentages; otherwise, both values are converted into a calc() function that is the sum of a length and a percentage (each possibly zero), and these calc() functions have each half interpolated as real numbers.
  • Canonical order order of appearance in the formal grammar of the values

See Using CSS flexible boxes for more properties and information.


/* 0 0 auto */
flex: none;

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

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

/* 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;


Defines the flex-grow of the flex item. See <number> for more details. Negative values are considered invalid. Defaults to 1 when omitted.
Defines the flex-shrink of the flex item. See <number> for more details. Negative values are considered invalid. Defaults to 1 when omitted.
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.
This keyword computes to 0 0 auto.

Formal syntax

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


<style type="text/css">
  #flex-container {

    /* flexbox specific properties */
    display: -webkit-flex;
    display: flex;
    -webkit-flex-direction: row;
    flex-direction: row;
  #flex-container .flex-item {
    border:1px solid #f00;

    /* flexbox specific properties */
    -webkit-flex: auto;
    flex: auto;

<div id="flex-container">
  <div class="flex-item">Flex Box 1</div>
  <div class="flex-item">Flex Box 2</div>


Specification Status Comment
CSS Flexible Box Layout Module
The definition of 'flex' in that specification.
Last Call Working Draft Initial definition

Browser compatibility

Feature Firefox (Gecko) Chrome Internet Explorer Opera Safari
Basic support 18.0 (18.0) (behind a pref) [1] [2]
20.0 (20.0) [2][3]
10.0-ms [3][4]
11.0 [3][4]
12.10 6.1-webkit
Feature Firefox Mobile (Gecko) Android IE Phone Opera Mobile Safari Mobile
Basic support ? 4.4 11 12.10 7.1-webkit

[1] To activate flexbox support for Firefox 18 and 19, the user has to change the about:config preference "layout.css.flexbox.enabled" to true.

[2] Multi-line flexbox are supported since Firefox 28.

[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.

[4] In Internet Explorer 10-11 (but not 12+), a flex declaration with a unitless value in itsflex-basis part is considered syntactically invalid and will thus be ignored. A workaround is to always include a unit in theflex-basis part of the flex shorthand value. See Flexbug #4 for more info.

See also