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

/* <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;


備註: min-width (en-US)max-width (en-US) 會覆寫 width.

預設值 (en-US)auto
Applies toall elements but non-replaced inline elements, table rows, and row groups
Percentagesrefer to the width of the containing block
Computed value (en-US)a percentage or auto or the absolute length
Animation typea length, percentage or calc();


The width property is specified as either:


<length> (en-US)

Defines the width as an absolute value.

<percentage> (en-US)

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> (en-US) or <percentage> (en-US) is applied to the element's border box.

content-box 實驗性質

If present, the preceding <length> (en-US) or <percentage> (en-US) is applied to the element's content box.


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



p.goldie {
  background: gold;
<p class="goldie">The Mozilla community produces a lot of great software.</p>


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


.percent {
  width: 20%;
  background-color: silver;
  border: 1px solid red;
<div class="percent">Width in percentage</div>


p.maxgreen {
  background: lightgreen;
  width: intrinsic; /* Safari/WebKit uses a non-standard name */
  width: -moz-max-content; /* Firefox/Gecko */
  width: -webkit-max-content; /* Chrome */
<p class="maxgreen">The Mozilla community produces a lot of great software.</p>


p.minblue {
  background: lightblue;
  width: -moz-min-content; /* Firefox */
  width: -webkit-min-content; /* Chrome */
<p class="minblue">The Mozilla community produces a lot of great software.</p>


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


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