Background

The

Flexbox Layout

(Flexible Box) module (currently a W3C Last Call Working Draft) aims at providing a more efficient way to lay out, align and distribute space among items in a container, even when their size is unknown and/or dynamic (thus the word “flex”).

The main idea behind the flex layout is to give the container the ability to alter its items’ width/height (and order) to best fill the available space (mostly to accommodate to all kind of display devices and screen sizes). A flex container expands items to fill available free space, or shrinks them to prevent overflow.

Most importantly, the flexbox layout is direction-agnostic as opposed to the regular layouts (block which is vertically-based and inline which is horizontally-based). While those work well for pages, they lack flexibility (no pun intended) to support large or complex applications (especially when it comes to orientation changing, resizing, stretching, shrinking, etc.).

Note: Flexbox layout is most appropriate to the components of an application, and small-scale layouts, while the Grid layout is intended for larger scale layouts.

Basics and Terminology

Properties for the Parent

display

This defines a flex container; inline or block depending on the given value. It enables a flex context for all its direct children.


<span class="token selector"><span class="token class">.container</span> </span><span class="token punctuation">{</span>
  <span class="token property">display</span><span class="token punctuation">:</span> flex<span class="token punctuation">;</span> <span class="token comment" spellcheck="true">/* or inline-flex */</span>
<span class="token punctuation">}</span>

Note that CSS columns have no effect on a flex container.

#flex-direction


This establishes the main-axis, thus defining the direction flex items are placed in the flex container. Flexbox is (aside from optional wrapping) a single-direction layout concept. Think of flex items as primarily laying out either in horizontal rows or vertical columns.


<span class="token selector"><span class="token class">.container</span> </span><span class="token punctuation">{</span>
  <span class="token property">flex-direction</span><span class="token punctuation">:</span> row | row-reverse | column | column-reverse<span class="token punctuation">;</span>
<span class="token punctuation">}</span>
  • row

    (default): left to right in

    ltr

    ; right to left in

    rtl
  • row-reverse

    : right to left in

    ltr

    ; left to right in

    rtl
  • column

    : same as

    row

    but top to bottom

  • column-reverse

    : same as

    row-reverse

    but bottom to top

#flex-wrap

By default, flex items will all try to fit onto one line. You can change that and allow the items to wrap as needed with this property.


<span class="token selector"><span class="token class">.container</span></span><span class="token punctuation">{</span>
  <span class="token property">flex-wrap</span><span class="token punctuation">:</span> nowrap | wrap | wrap-reverse<span class="token punctuation">;</span>
<span class="token punctuation">}</span>
  • nowrap

    (default): all flex items will be on one line

  • wrap

    : flex items will wrap onto multiple lines, from top to bottom.

  • wrap-reverse

    : flex items will wrap onto multiple lines from bottom to top.

There are some visual demos of

flex-wrap

here.

#flex-flow (Applies to: parent flex container element)

This is a shorthand

flex-direction

and

flex-wrap

properties, which together define the flex container’s main and cross axes. Default is

row nowrap

.


<span class="token property">flex-flow</span><span class="token punctuation">:</span> &lt;‘flex-direction’&gt; || &lt;‘flex-wrap’&gt;

#justify-content


This defines the alignment along the main axis. It helps distribute extra free space left over when either all the flex items on a line are inflexible, or are flexible but have reached their maximum size. It also exerts some control over the alignment of items when they overflow the line.


<span class="token selector"><span class="token class">.container</span> </span><span class="token punctuation">{</span>
  <span class="token property">justify-content</span><span class="token punctuation">:</span> flex-start | flex-end | center | space-between | space-around | space-evenly<span class="token punctuation">;</span>
<span class="token punctuation">}</span>
  • flex-start

    (default): items are packed toward the start line

  • flex-end

    : items are packed toward to end line

  • center

    : items are centered along the line

  • space-between

    : items are evenly distributed in the line; first item is on the start line, last item on the end line

  • space-around

    : items are evenly distributed in the line with equal space around them. Note that visually the spaces aren’t equal, since all the items have equal space on both sides. The first item will have one unit of space against the container edge, but two units of space between the next item because that next item has its own spacing that applies.

  • space-evenly

    : items are distributed so that the spacing between any two items (and the space to the edges) is equal.

#align-items


This defines the default behaviour for how flex items are laid out along the cross axis on the current line. Think of it as the

justify-content

version for the cross-axis (perpendicular to the main-axis).


<span class="token selector"><span class="token class">.container</span> </span><span class="token punctuation">{</span>
  <span class="token property">align-items</span><span class="token punctuation">:</span> flex-start | flex-end | center | baseline | stretch<span class="token punctuation">;</span>
<span class="token punctuation">}</span>
  • flex-start

    : cross-start margin edge of the items is placed on the cross-start line

  • flex-end

    : cross-end margin edge of the items is placed on the cross-end line

  • center

    : items are centered in the cross-axis

  • baseline

    : items are aligned such as their baselines align

  • stretch

    (default): stretch to fill the container (still respect min-width/max-width)

#align-content


This aligns a flex container’s lines within when there is extra space in the cross-axis, similar to how

justify-content

aligns individual items within the main-axis.

Note: this property has no effect when there is only one line of flex items.


<span class="token selector"><span class="token class">.container</span> </span><span class="token punctuation">{</span>
  <span class="token property">align-content</span><span class="token punctuation">:</span> flex-start | flex-end | center | space-between | space-around | stretch<span class="token punctuation">;</span>
<span class="token punctuation">}</span>
  • flex-start

    : lines packed to the start of the container

  • flex-end

    : lines packed to the end of the container

  • center

    : lines packed to the center of the container

  • space-between

    : lines evenly distributed; the first line is at the start of the container while the last one is at the end

  • space-around

    : lines evenly distributed with equal space around each line

  • stretch

    (default): lines stretch to take up the remaining space

Properties for the Children

order


By default, flex items are laid out in the source order. However, the

order

property controls the order in which they appear in the flex container.


<span class="token selector"><span class="token class">.item</span> </span><span class="token punctuation">{</span>
  <span class="token property">order</span><span class="token punctuation">:</span> &lt;integer&gt;<span class="token punctuation">;</span> <span class="token comment" spellcheck="true">/* default is 0 */</span>
<span class="token punctuation">}</span>

#flex-grow


This defines the ability for a flex item to grow if necessary. It accepts a unitless value that serves as a proportion. It dictates what amount of the available space inside the flex container the item should take up.

If all items have

flex-grow

set to 1, the remaining space in the container will be distributed equally to all children. If one of the children has a value of 2, the remaining space would take up twice as much space as the others (or it will try to, at least).


<span class="token selector"><span class="token class">.item</span> </span><span class="token punctuation">{</span>
  <span class="token property">flex-grow</span><span class="token punctuation">:</span> &lt;number&gt;<span class="token punctuation">;</span> <span class="token comment" spellcheck="true">/* default 0 */</span>
<span class="token punctuation">}</span>

Negative numbers are invalid.

#flex-shrink

This defines the ability for a flex item to shrink if necessary.


<span class="token selector"><span class="token class">.item</span> </span><span class="token punctuation">{</span>
  <span class="token property">flex-shrink</span><span class="token punctuation">:</span> &lt;number&gt;<span class="token punctuation">;</span> <span class="token comment" spellcheck="true">/* default 1 */</span>
<span class="token punctuation">}</span>

Negative numbers are invalid.

#flex-basis

This defines the default size of an element before the remaining space is distributed. It can be a length (e.g. 20%, 5rem, etc.) or a keyword. The

auto

keyword means “look at my width or height property” (which was temporarily done by the

main-size

keyword until deprecated). The

content

keyword means “size it based on the item’s content” – this keyword isn’t well supported yet, so it’s hard to test and harder to know what its brethren

max-content

,

min-content

, and

fit-content

do.


<span class="token selector"><span class="token class">.item</span> </span><span class="token punctuation">{</span>
  <span class="token property">flex-basis</span><span class="token punctuation">:</span> &lt;length&gt; | auto<span class="token punctuation">;</span> <span class="token comment" spellcheck="true">/* default auto */</span>
<span class="token punctuation">}</span>

If set to

0

, the extra space around content isn’t factored in. If set to

auto

, the extra space is distributed based on its

flex-grow

value. See this graphic.

#flex

This is the shorthand for

flex-grow,
flex-shrink

and

flex-basis

combined. The second and third parameters (

flex-shrink

and

flex-basis

) are optional. Default is

0 1 auto

.


<span class="token selector"><span class="token class">.item</span> </span><span class="token punctuation">{</span>
  <span class="token property">flex</span><span class="token punctuation">:</span> none | [ &lt;<span class="token string">'flex-grow'</span>&gt; &lt;<span class="token string">'flex-shrink'</span>&gt;? || &lt;<span class="token string">'flex-basis'</span>&gt; ]
<span class="token punctuation">}</span>

It is recommended that you use this shorthand property rather than set the individual properties. The short hand sets the other values intelligently.

#align-self


This allows the default alignment (or the one specified by

align-items

) to be overridden for individual flex items.

Please see the

align-items

explanation to understand the available values.


<span class="token selector"><span class="token class">.item</span> </span><span class="token punctuation">{</span>
  <span class="token property">align-self</span><span class="token punctuation">:</span> auto | flex-start | flex-end | center | baseline | stretch<span class="token punctuation">;</span>
<span class="token punctuation">}</span>

Note that

float

,

clear

and

vertical-align

have no effect on a flex item.

Browser Support

Broken up by “version” of flexbox:

  • (new) means the recent syntax from the specification (e.g.
    display: flex;

    )

  • (tweener) 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
20- (old)
21+ (new)
3.1+ (old)
6.1+ (new)
2-21 (old)
22+ (new)
12.1+ (new) 10 (tweener)
11+ (new)
2.1+ (old)
4.4+ (new)
3.2+ (old)
7.1+ (new)

Blackberry browser 10+ supports the new syntax.

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

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