flex

This property is a shorthand for the following CSS properties:

• flex-grow
• flex-shrink
• flex-basis

/* Keyword value */
flex: none; /* 0 0 auto */

/* One value, unitless number: flex-grow
flex-basis is then equal to 0%. */
flex: 2; /* 2 1 0% */

/* One value, width/height: flex-basis */
flex: auto; /* 1 1 auto */
flex: 10em; /* 1 1 10em */
flex: 30%;
flex: min-content;

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

/* Two values: flex-grow | flex-shrink */
flex: 2 2; /* 2 2 0% */

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

/* Global values */
flex: inherit;
flex: initial; /* 0 1 auto */
flex: revert;
flex: revert-layer;
flex: unset;

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

• One-value syntax: the value must be one of:
  • a valid value for <flex-grow>: then, in all the browsers, the shorthand expands to flex: <flex-grow> 1 0%. However the specification says it should expand to flex: <flex-grow> 1 0.
  • a valid value for <flex-basis>: then the shorthand expands to flex: 1 1 <flex-basis>.
  • the keyword none or one of the global keywords.
• Two-value syntax:
  • The first value must be a valid value for flex-grow.
  • The second value must be one of:
    • a valid value for flex-shrink: then, in all the browsers, the shorthand expands to flex: <flex-grow> <flex-shrink> 0%.
    • a valid value for flex-basis: then the shorthand expands to flex: <flex-grow> 1 <flex-basis>.
• Three-value syntax: the values must be in the following order:
  1. a valid value for flex-grow.
  2. a valid value for flex-shrink.
  3. a valid value for flex-basis.

<'flex-grow'>

Defines the flex-grow of the flex item. Negative values are considered invalid. Defaults to 1 when omitted. (initial is 0)

<'flex-shrink'>

Defines the flex-shrink of the flex item. Negative values are considered invalid. Defaults to 1 when omitted. (initial is 1)

<'flex-basis'>

Defines the flex-basis of the flex item. Defaults to 0% when omitted. The initial value is 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.

Commonly desired flexbox effects can be achieved using the following flex values:

• initial: Flex item doesn't grow but can shrink. This default value expands to flex: 0 1 auto. The item is sized according to its width or height properties, depending on the flex-direction. If there is negative available space, the item will shrink to its minimum size to fit within the container but will not grow to absorb any positive space available in the flex container.
• auto: Flex item can grow and shrink. This value expands to flex: 1 1 auto. The item is sized according to its width or height properties, depending on the flex-direction, but grows to absorb available positive space in the flex container or shrink down to its minimum size to fit the container in the case of negative space. The flex item is fully flexible.
• none: The flex item neither grows nor shrinks. This value expands to flex: 0 0 auto. The item is sized according to its width or height properties, depending on the direction of the flex container. The flex item is fully inflexible.
• flex: <number [1,∞]>: The flex item's main size will be proportional to the number set. This value expands to flex: <number> 1 0. This sets the flex-basis to zero and makes the flex item flexible. The item will be at least as wide or tall as its minimum size, with the container's positive available space being proportionally distributed based on the growth factors of this item and its sibling flex items. If all the flex items use this pattern, all will be sized in proportion to their numeric values.

  Warning: The browsers use flex-basis value 0% when the flex-basis is not specified in a flex value. This is not the same as flex-basis value 0 which is what the specification says. This may affect flex layout in some cases. See this effect demonstrated in the Flex-basis 0 versus 0% example.

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:

<div class="flex-container">
  <div class="item auto">auto</div>
  <div class="item auto">auto</div>
  <div class="item auto">auto</div>
</div>

<div class="flex-container">
  <div class="item auto">auto</div>
  <div class="item initial">initial</div>
  <div class="item initial">initial</div>
</div>

<div class="flex-container">
  <div class="item auto">auto</div>
  <div class="item auto">auto</div>
  <div class="item none">none</div>
</div>

<div class="flex-container">
  <div class="item initial">initial</div>
  <div class="item none">none</div>
  <div class="item none">none</div>
</div>

<div class="flex-container">
  <div class="item four">4</div>
  <div class="item two">2</div>
  <div class="item one">1</div>
</div>

* {
  box-sizing: border-box;
}

.flex-container {
  background-color: #f4f7f8;
  resize: horizontal;
  overflow: hidden;
  display: flex;
  margin: 1em;
}

.item {
  margin: 1em;
  padding: 0.5em;
  width: 110px;
  min-width: 0;
  background-color: #1b5385;
  color: white;
  font-family: monospace;
  font-size: 13px;
}

.initial {
  flex: initial;
}

.auto {
  flex: auto;
}

.none {
  flex: none;
}

.four {
  flex: 4;
}

.two {
  flex: 2;
}

.one {
  flex: 1;
}

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

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

<flex-grow> = 
  <number [0,∞]>  

<flex-shrink> = 
  <number [0,∞]>  

<flex-basis> = 
  content    |
  <'width'>  

<width> = 
  auto                                      |
  <length-percentage [0,∞]>                 |
  min-content                               |
  max-content                               |
  fit-content( <length-percentage [0,∞]> )  |
  <calc-size()>                             |
  <anchor-size()>                           

<length-percentage> = 
  <length>      |
  <percentage>  

<calc-size()> = 
  calc-size( <calc-size-basis> , <calc-sum> )  

<anchor-size()> = 
  anchor-size( [ <anchor-element> || <anchor-size> ]? , <length-percentage>? )  

<calc-size-basis> = 
  <intrinsic-size-keyword>  |
  <calc-size()>             |
  any                       |
  <calc-sum>                

<calc-sum> = 
  <calc-product> [ [ '+' | '-' ] <calc-product> ]*  

<anchor-element> = 
  <dashed-ident>  

<anchor-size> = 
  width        |
  height       |
  block        |
  inline       |
  self-block   |
  self-inline  

<calc-product> = 
  <calc-value> [ [ '*' | '/' ] <calc-value> ]*  

<calc-value> = 
  <number>        |
  <dimension>     |
  <percentage>    |
  <calc-keyword>  |
  ( <calc-sum> )  

<calc-keyword> = 
  e          |
  pi         |
  infinity   |
  -infinity  |
  NaN        

This example shows how a flex item with flex: auto grows to absorb any free space in the container.

HTML

<div id="flex-container">
  <div id="flex-auto">
    flex: auto (click to remove/add the `flex: initial` box)
  </div>
  <div id="default">flex: initial</div>
</div>

CSS

body * {
  padding: 1rem;
  text-select: none;
  box-sizing: border-box;
  font-family: Consolas, Arial, sans-serif;
}

#flex-container {
  border: 2px dashed gray;
  display: flex;
}

#flex-auto {
  cursor: pointer;
  background-color: wheat;

  flex: auto;
}

#default {
  background-color: lightblue;
}

JavaScript

const flexAutoItem = document.getElementById("flex-auto");
const defaultItem = document.getElementById("default");
flexAutoItem.addEventListener("click", () => {
  defaultItem.style.display =
    defaultItem.style.display === "none" ? "block" : "none";
});

Result

The flex container contains two flex items:

• The #flex-auto item has a flex value of auto. The auto value expands to 1 1 auto, i.e. the item is allowed to expand.
• The #default item has no flex value set so it defaults to the initial value. The initial value expands to 0 1 auto, i.e. the item is not allowed to expand.

The #default item takes up as much space as its width requires, but does not expand to take up any more space. All the remaining space is taken up by the #flex-auto item.

When you click the #flex-auto item, we set the #default item's display property to none, removing it from the layout. The #flex-auto item then expands to occupy all the available space in the container. Clicking the #flex-auto item again adds the #default item back to the container.

• Basic concepts of flexbox
• Controlling ratios of flex items along the main axis
• CSS flexible box layout module