flex - CSS: Cascading Style Sheets 编辑

The flex CSS shorthand property sets how a flex item will grow or shrink to fit the space available in its flex container.

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The source for this interactive example is stored in a GitHub repository. If you'd like to contribute to the interactive examples project, please clone https://github.com/mdn/interactive-examples and send us a pull request.

Constituent properties

This property is a shorthand for the following CSS properties:

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

Syntax

/* Keyword values */
flex: auto;
flex: initial;
flex: none;

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

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

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

• One-value syntax: the value must be one of:
  • a <number>: In this case it is interpreted as flex: <number> 1 0; the <flex-shrink> value is assumed to be 1 and the <flex-basis> value is assumed to be 0.
  • one of the keywords: none, auto, or initial.
• Two-value syntax:
  • The first value must be:
    • a <number> and it is interpreted as <flex-grow>.
  • The second value must be one of:
    • a <number>: then it is interpreted as <flex-shrink>.
    • a valid value for width: then it is interpreted as <flex-basis>.
• Three-value syntax: the values must be in the following order:
  1. a <number> for <flex-grow>.
  2. a <number> for <flex-shrink>.
  3. a valid value for width for <flex-basis>.

Values

initial

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".

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".

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".

<'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. A preferred size of 0 must have a unit to avoid being interpreted as a flexibility. Defaults to 0 when omitted. (initial is auto)

Description

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 minimum content size. To change this, set the item's min-width or min-height.

Formal definition

Initial value	as each of the properties of the shorthand: flex-grow: 0 flex-shrink: 1 flex-basis: auto
Applies to	flex items, including in-flow pseudo-elements
Inherited	no
Computed value	as each of the properties of the shorthand: flex-grow: as specified flex-shrink: as specified flex-basis: as specified, but with relative lengths converted into absolute lengths
Animation type	as each of the properties of the shorthand: flex-grow: a number flex-shrink: a number flex-basis: a length, percentage or calc();

Formal syntax

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

Examples

Setting flex: auto

HTML

<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>

CSS

#flex-container {
  display: flex;
  flex-direction: row;
}

#flex-container > .flex-item {
  flex: auto;
}

#flex-container > .raw-item {
  width: 5rem;
}

var flex = document.getElementById("flex");
var raw = document.getElementById("raw");
flex.addEventListener("click", function() {
  raw.style.display = raw.style.display == "none" ? "block" : "none";
});

#flex-container {
  width: 100%;
  font-family: Consolas, Arial, sans-serif;
}

#flex-container > div {
  border: 1px solid #f00;
  padding: 1rem;
}

#flex-container > .raw-item {
  border: 1px solid #000;
}

Result

Specifications

Specification	Status	Comment
CSS Flexible Box Layout Module The definition of 'flex' in that specification.	Candidate Recommendation	Initial definition

Browser compatibility

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The compatibility table on this page is generated from structured data. If you'd like to contribute to the data, please check out https://github.com/mdn/browser-compat-data and send us a pull request.

See also

• CSS Flexbox Guide: Basic Concepts of Flexbox
• CSS Flexbox Guide: Controlling Ratios of flex items along the main axis