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.

/* <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;

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

預設值auto
Applies toall elements but non-replaced inline elements, table rows, and row groups
繼承與否no
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();

Syntax

The width property is specified as either:

Values

<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
If present, the preceding <length> (en-US) or <percentage> (en-US) is applied to the element's border box.
content-box
If present, the preceding <length> (en-US) or <percentage> (en-US) is applied to the element's content box.
auto
The browser will calculate and select a width for the specified element.
fill
Use the fill-available inline size or fill-available block size, as appropriate to the writing mode.
max-content
The intrinsic preferred width.
min-content
The intrinsic minimum width.
available
The containing block width minus horizontal margin, border and padding.
fit-content
The larger of:
  • the intrinsic minimum width
  • the smaller of the intrinsic preferred width and the available width

Formal syntax

auto | (en-US) <length> (en-US) | (en-US) <percentage> (en-US) | (en-US) min-content | (en-US) max-content | (en-US) fit-content | (en-US) fit-content(<length-percentage>)

where
<length-percentage> = <length> (en-US) | (en-US) <percentage> (en-US)

範例

預設寬度

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

像素和相對大小

.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>

百分比

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

內容最大值

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>

內容最小值

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>

規範

Specification Status Comment
CSS Box Model
The definition of 'width' in that specification.
Candidate Recommendation Added the max-content, min-content, available, fit-content, border-box, content-box keywords.
CSS Transitions
The definition of 'width' in that specification.
Working Draft Lists width as animatable.
CSS Level 2 (Revision 1)
The definition of 'width' in that specification.
Recommendation Precises on which element it applies to.
CSS Level 1
The definition of 'width' in that specification.
Recommendation Initial definition.
CSS Box Sizing Module Level 3
The definition of 'width' in that specification.
Working Draft Adds new sizing keywords for width and height.

瀏覽器相容性

BCD tables only load in the browser

參考