width

Baseline Widely available *

This feature is well established and works across many devices and browser versions. It’s been available across browsers since ⁨2015年7月⁩.

* Some parts of this feature may have varying levels of support.

The width CSS property specifies the width of an element. By default, the property defines the width of the content area. If box-sizing is set to border-box, however, it instead determines the width of the border area.

css

/* <length> values */
width: 300px;
width: 25em;

/* <percentage> value */
width: 75%;

/* Keyword values */
width: 25em border-box;
width: 75% content-box;
width: max-content;
width: min-content;
width: available;
width: fit-content;
width: auto;

/* Global values */
width: inherit;
width: initial;
width: unset;

Examples

<div class="grid">
  <div class="col">
    <div class="cell">
      &lt;length&gt; values
      <p class="w1">width: 150px</p>
      <p class="w2">width: 20em</p>
      <p
        class="w3 warning"
        title="this feature is experimental and might not work in your browser">
        width: 20em content-box
      </p>
      <p
        class="w4 warning"
        title="this feature is experimental and might not work in your browser">
        width: 20em border-box
      </p>
    </div>
    <div class="cell">
      &lt;percentage&gt; values
      <p class="w5">width: 75%</p>
      <p
        class="w6 warning"
        title="this feature is experimental and might not work in your browser">
        width: 75% content-box
      </p>
      <p
        class="w7 warning"
        title="this feature is experimental and might not work in your browser">
        width: 75% border-box
      </p>
    </div>
    <div class="cell">
      Keyword values
      <p>width: auto</p>
      <p
        class="w8 warning"
        title="this feature is experimental and might not work in your browser">
        width: max-content
      </p>
      <p
        class="w9 warning"
        title="this feature is experimental and might not work in your browser">
        width: min-content
      </p>
      <p
        class="w10 warning"
        title="this feature is experimental and might not work in your browser">
        width: available
      </p>
      <p
        class="w11 warning"
        title="this feature is experimental and might not work in your browser">
        width: fit-content
      </p>
    </div>
  </div>
</div>

html,
body {
  height: 100%;
  box-sizing: border-box;
}

.grid {
  width: 100%;
  height: 100%;
  display: flex;
  background: #eee;
  font: 1em monospace;
}

.col {
  display: flex;
  flex: 1 auto;
  flex-direction: column;
}

.cell {
  margin: 0.5em;
  padding: 0.5em;
  background-color: #fff;
  overflow: hidden;
  text-align: left;
  font-style: italic;
}

p {
  font-size: 1rem;
  font-style: normal;
  background: #e4f0f5;
  padding: 0.5em;
  margin: 0.5em;
}

.warning {
  background: #e4e4e4;
}

.warning:before {
  content: "⚠️ ";
}

/*  values */
.w1 {
  width: 150px;
}
.w2 {
  width: 20em;
}
.w3 {
  width: 20em content-box;
}
.w4 {
  width: 20em border-box;
}

/*  value */
.w5 {
  width: 75%;
}
.w6 {
  width: 75% content-box;
}
.w7 {
  width: 75% border-box;
}

/* Keyword values (mostly experimental) */
.w8 {
  width: max-content;
}
.w9 {
  width: min-content;
}
.w10 {
  width: available;
}
.w11 {
  width: fit-content;
}

備註：min-width 和 max-width 會覆寫 width.

預設值	`auto`
適用於	所有元素，除了非替換的行內元素、表格列和列群組
繼承與否	否
百分比	指包含方塊的寬度
Computed value	百分比、`auto` 或絕對長度
動畫類型	a length，百分比或 calc();

Syntax

The width property is specified as either:

one of the following keyword values: available, min-content, max-content, fit-content, auto.
a <length> or a <percentage>. This may optionally be followed by one of the following keywords: border-box, content-box.

Values

<length>

Defines the width as an absolute value.

<percentage>

Defines the width as a percentage of the containing block's width. If the width of the containing block depends on the width of the element, the resulting layout is undefined.

border-box 實驗性質

If present, the preceding <length> or <percentage> is applied to the element's border box.

content-box 實驗性質

If present, the preceding <length> or <percentage> is applied to the element's content box.

auto

The browser will calculate and select a width for the specified element.

fill 實驗性質

Use the fill-available inline size or fill-available block size, as appropriate to the writing mode.

max-content 實驗性質

The intrinsic preferred width.

min-content 實驗性質

The intrinsic minimum width.

available 實驗性質

The containing block width minus horizontal margin, border and padding.

fit-content 實驗性質

The larger of: the intrinsic minimum width

the smaller of the intrinsic preferred width and the available width

Formal syntax

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

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

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

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

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

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

<anchor-name> = 
  <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

範例

預設寬度

css

p.goldie {
  background: gold;
}

html

<p class="goldie">The Mozilla community produces a lot of great software.</p>

像素和相對大小

css

.px_length {
  width: 200px;
  background-color: red;
  color: white;
  border: 1px solid black;
}

.em_length {
  width: 20em;
  background-color: white;
  color: red;
  border: 1px solid black;
}

html

<div class="px_length">Width measured in px</div>
<div class="em_length">Width measured in em</div>

百分比

css

.percent {
  width: 20%;
  background-color: silver;
  border: 1px solid red;
}

html

<div class="percent">Width in percentage</div>

內容最大值

css

p.maxgreen {
  background: lightgreen;
  width: intrinsic; /* Safari/WebKit uses a non-standard name */
  width: -moz-max-content; /* Firefox/Gecko */
  width: -webkit-max-content; /* Chrome */
}

html

<p class="maxgreen">The Mozilla community produces a lot of great software.</p>

內容最小值

css

p.minblue {
  background: lightblue;
  width: -moz-min-content; /* Firefox */
  width: -webkit-min-content; /* Chrome */
}

html

<p class="minblue">The Mozilla community produces a lot of great software.</p>

規範

Specification
CSS Box Sizing Module Level 4 # sizing-values

瀏覽器相容性

參見

box model, height, box-sizing, min-width, max-width

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This page was last modified on ⁨2025年11月10日⁩ by MDN contributors.

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