flex

The flex property is a sub-property of the Flexible Box Layout module.

This is the shorthand for flex-grow, flex-shrink and flex-basis. The second and third parameters (flex-shrink and flex-basis) are optional.

Syntax

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

.flex-item {

  /* this */
  flex: 1 100px;

  /* is the same as */
  flex-grow: 1;
  flex-basis: 100px;

  /* and it leaves the flex-shrink property alone, which would be */
  flex-shrink: inherit; /* defaults to 1 */

}

Here’s the scoop on what the values map to depending on how many values you give it:

flex: none /* value 'none' case */
flex: <'flex-grow'> /* One value syntax, variation 1 */
flex: <'flex-basis'> /* One value syntax, variation 2 */
flex: <'flex-grow'> <'flex-basis'> /* Two values syntax, variation 1 */
flex: <'flex-grow'> <'flex-shrink'> /* Two values syntax, variation 2 */
flex: <'flex-grow'> <'flex-shrink'> <'flex-basis'> /* Three values syntax */
flex: inherit

Common values for flex

flex: 0 auto;

This is the same as flex: initial; and the shorthand for the default value: flex: 0 1 auto. It sizes the item based on its width/height properties (or its content if not set).

It makes the flex item inflexible when there is some free space left, but allows it to shrink to its minimum when there is not enough space. The alignment abilities or auto margins can be used to align flex items along the main axis.

flex: auto;

This is equivalent to flex: 1 1 auto. Beware, this is not the default value. It sizes the item based on its width/height properties, but makes it fully flexible so that they absorb any extra space along the main axis.

If all items are either flex: auto, flex: initial, or flex: none, any remaining space after the items have been sized will be distributed evenly to the items with flex: auto.

flex: none;

This is equivalent to flex: 0 0 auto. It sizes the item according to its width/height properties, but makes it fully inflexible.

This is similar to flex: initial except it is not allowed to shrink, even in an overflow situation.

flex: <positive-number>

Equivalent to flex: 1 0px. It makes the flex item flexible and sets the flex basis to zero, resulting in an item that receives the specified proportion of the remaining space.

If all items in the flex container use this pattern, their sizes will be proportional to the specified flex factor.

Demo

The following demo shows a very simple layout with Flexbox thanks to the flex property:

Here is the revelant bit of code:

.header,
.footer  { flex: 1 100%; }
.sidebar { flex: 1; }
.main    { flex: 2; }

First, we’ve authorized flex items to be displayed on multiple rows with flex-flow: row wrap.

Then we tell to both the header and the footer to take 100% of the current viewport width, no matter what.

And the main content and both sidebars will share the same row, sharing the remaining space as follow: 66% (2/(1+2)) for the main content, 33% (1/(1+2)) for the sidebar.

Related Properties

• flex-grow
• flex-shrink
• flex-basis
• align-self
• order

Other Resources

• flex in the spec
• flex at MDN
• Advanced cross-browser flexbox
• A guide to Flexbox
• Using Flexbox
• Old Flexbox and new Flexbox

Browser Support

• (modern) means the recent syntax from the specification (e.g. display: flex;)
• (hybrid) means an odd unofficial syntax from 2011 (e.g. display: flexbox;)
• (old) means the old syntax from 2009 (e.g. display: box;)

Chrome	Safari	Firefox	Opera	IE	Android	iOS
21+ (modern) 20- (old)	3.1+ (old)	2-21 (old) 22+ (new)	12.1+ (modern)	10+ (hybrid)	2.1+ (old)	3.2+ (old)

Blackberry browser 10+ supports the new syntax.

For more information about how to mix syntaxes in order to get the best browser support, please refer to this article (CSS-Tricks) or this article (DevOpera).