width

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`
Applies to	all elements but non-replaced inline elements, table rows, and row groups
繼承與否	no
Percentages	refer to the width of the containing block
Computed value	a percentage or `auto` or the absolute length
Animation type	a length, percentage or 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()>                           

<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

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 # width-height-keywords
CSS Box Sizing Module Level 4 # sizing-values

瀏覽器相容性

BCD tables only load in the browser

參考

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