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<length> CSS data type represents a distance value. Lengths can be used in numerous CSS properties, such as width , height , margin , padding , border-width , font-size ，和 text-shadow .

注意： Although <percentage> values are also CSS dimensions, and are usable in some of the same properties that accept <length> values, they are not themselves <length> values. See <length-percentage> .

句法

<length> data type consists of a <number> followed by one of the units listed below. As with all CSS dimensions, there is no space between the unit literal and the number. The length unit is optional after the number 0 .

注意： Some properties allow negative <length> s, while others do not.

Units

Relative length units

Relative lengths represent a measurement in terms of some other distance. Depending on the unit, this can be the size of a specific character, the line height , or the size of the viewport .

Font-relative lengths

Font-relative lengths define the <length> value in terms of the size of a particular character or font attribute in the font currently in effect in an element or its parent.

注意： These units, especially em and rem , are often used to create scalable layouts, which maintain the vertical rhythm of the page even when the user changes the font size.

cap
Represents the "cap height" (nominal height of capital letters) of the element’s font .
ch
Represents the width, or more precisely the advance measure, of the glyph "0" (zero, the Unicode character U+0030) in the element's font .

In the cases where it is impossible or impractical to determine the measure of the “0” glyph, it must be assumed to be 0.5em wide by 1em tall.
em
Represents the calculated font-size of the element. If used on the font-size property itself, it represents the inherited font-size of the element.
ex
Represents the x-height of the element's font . On fonts with the "x" letter, this is generally the height of lowercase letters in the font; 1ex ≈ 0.5em in many fonts.
ic

Equal to the used advance measure of the "水" glyph (CJK water ideograph, U+6C34), found in the font used to render it.

lh
Equal to the computed value of the line-height property of the element on which it is used, converted to an absolute length.
rem
Represents the font-size of the root element (typically <html> ). When used within the root element font-size , it represents its initial value (a common browser default is 16px , but user-defined preferences may modify this).
rlh
Equal to the computed value of the line-height property on the root element (typically <html> ), converted to an absolute length. When used on the font-size or line-height properties of the root element, it refers to the properties' initial value.
Viewport-percentage lengths

Viewport-percentage lengths define the <length> value relative to the size of the viewport , i.e., the visible portion of the document. Viewport lengths are invalid in @page declaration blocks.

vh
Equal to 1% of the height of the viewport's initial 包含块 .
vw
Equal to 1% of the width of the viewport's initial 包含块 .
vi
Equal to 1% of the size of the initial 包含块 , in the direction of the root element’s inline axis .
vb
Equal to 1% of the size of the initial 包含块 , in the direction of the root element’s block axis .
vmin
Equal to the smaller of vw and vh .
vmax
Equal to the larger of vw and vh .

Absolute length units

Absolute length units represent a physical measurement when the physical properties of the output medium are known, such as for print layout. This is done by anchoring one of the units to a physical unit, and then defining the others relative to it. The anchor is done differently for low-resolution devices, such as screens, versus high-resolution devices, such as printers.

For low-dpi devices, the unit px represents the physical reference pixel ; other units are defined relative to it. Thus, 1in is defined as 96px , which equals 72pt . The consequence of this definition is that on such devices, dimensions described in inches ( in ), centimeters ( cm ), or millimeters ( mm ) don't necessary match the size of the physical unit with the same name.

For high-dpi devices, inches ( in ), centimeters ( cm ), and millimeters ( mm ) are the same as their physical counterparts. Therefore, the px unit is defined relative to them (1/96 of 1 inch).

注意： Many users increase their 用户代理 's default font size to make text more legible. Absolute lengths can cause accessibility problems, since they are fixed and do not scale according to user settings. For this reason, prefer relative lengths (such as em or rem ) when setting font-size .

px
One pixel. For screen displays, it traditionally represents one device pixel (dot). However, for printers and high-resolution screens , one CSS pixel implies multiple device pixels. 1px = 1/96th of 1in .
cm
One centimeter. 1cm = 96px/2.54 .
mm
One millimeter. 1mm = 1/10th of 1cm .
Q
One quarter of a millimeter. 1Q = 1/40th of 1cm .
in
One inch. 1in = 2.54cm = 96px .
pc
One pica. 1pc = 12pt = 1/6th of 1in .
pt
One point. 1pt = 1/72nd of 1in .
mozmm , removed in Firefox 59

An experimental unit that attempts to render at exactly one millimeter regardless of the size or resolution of the display. This is rarely actually what you want, but may be useful in particular for mobile devices.

插值

When animated, values of the <length> data type are interpolated as real, floating-point numbers. The interpolation happens on the calculated value. The speed of the interpolation is determined by the timing function associated with the animation.

范例

Length unit comparison

The following demo provides you with an input field in which you can enter a <length> value (e.g. 300px , 50% , 30vw ) to set the width of a result bar that will appear below it once you've pressed 返回 .

This allows you to compare and contrast the effect of different length units.

HTML 

<div class="outer">
  <div class="input-container">
    <label>Enter width:</label>
    <input type="text" id="length">
  </div>
  <div class="inner">
  </div>
</div>
<div class="results">
</div>

CSS 

html {
  font-family: sans-serif;
  font-weight: bold;
  box-sizing: border-box;
}
.outer {
  width: 100%;
  height: 50px;
  background-color: #eee;
  position: relative;
}
.inner {
  height: 50px;
  background-color: #999;
  box-shadow: inset 3px 3px 5px rgba(255,255,255,0.5),
              inset -3px -3px 5px rgba(0,0,0,0.5);
}
.result {
  height: 20px;
  background-color: #999;
  box-shadow: inset 3px 3px 5px rgba(255,255,255,0.5),
              inset -3px -3px 5px rgba(0,0,0,0.5);
  background-color: orange;
  display: flex;
  align-items: center;
  margin-top: 10px;
}
.result code {
  position: absolute;
  margin-left: 20px;
}
.results {
  margin-top: 10px;
}
.input-container {
  position: absolute;
  display: flex;
  justify-content: flex-start;
  align-items: center;
  height: 50px;
}
label {
  margin: 0 10px 0 20px;
}

JavaScript 

const inputDiv = document.querySelector('.inner');
const inputElem = document.querySelector('input');
const resultsDiv = document.querySelector('.results');
inputElem.addEventListener('change', () => {
  inputDiv.style.width = inputElem.value;
  const result = document.createElement('div');
  result.className = 'result';
  result.style.width = inputElem.value;
  result.innerHTML = `<code>width: ${inputElem.value}</code>`;
  resultsDiv.appendChild(result);
  inputElem.value = '';
  inputElem.focus();
})

结果

规范

规范 状态 注释
CSS Values and Units Module Level 4
The definition of '<length>' in that specification. 		编者草案 添加 vi , vb , ic , lh ，和 rlh units.
CSS Values and Units Module Level 3
The definition of '<length>' in that specification. 		候选推荐 添加 ch , rem , vw , vh , vmin , vmax ，和 Q units.
CSS Level 2 (Revision 1)
The definition of '<length>' in that specification. 		推荐 Explicit definition of the em , pt , pc ，和 px units.
CSS Level 1
The definition of '<length>' in that specification. 		推荐 Initial definition. Implicit definition of the em , pt , pc ，和 px units.

浏览器兼容性

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. 更新 GitHub 上的兼容性数据
Desktop Mobile
Chrome Edge Firefox Internet Explorer Opera Safari Android webview Chrome for Android Firefox for Android Opera for Android Safari on iOS Samsung Internet
<length> Chrome 完整支持 1 Edge 完整支持 12 Firefox 完整支持 1 IE 完整支持 3 Opera 完整支持 3.5 Safari 完整支持 1 WebView Android 完整支持 ≤37 Chrome Android 完整支持 18 Firefox Android 完整支持 4 Opera Android 完整支持 10.1 Safari iOS 完整支持 1 Samsung Internet Android 完整支持 1.0
Q unit Chrome 完整支持 63 Edge 完整支持 79 Firefox 完整支持 49 IE 不支持 No Opera 完整支持 50 Safari 不支持 No WebView Android 完整支持 63 Chrome Android 完整支持 63 Firefox Android 完整支持 49 Opera Android 完整支持 46 Safari iOS 不支持 No Samsung Internet Android 完整支持 8.0
cap unit Chrome 不支持 No Edge 不支持 No Firefox 不支持 No IE 不支持 No Opera 不支持 No Safari 不支持 No WebView Android 不支持 No Chrome Android 不支持 No Firefox Android 不支持 No Opera Android 不支持 No Safari iOS 不支持 No Samsung Internet Android 不支持 No
ch unit Chrome 完整支持 27 Edge 完整支持 12 Firefox 完整支持 1 注意事项
完整支持 1 注意事项
注意事项 From Firefox 1 to Firefox 3, ch was the width of the M character.
注意事项 From Firefox 1 to Firefox 3, ch did not work with border-width and outline-width CSS properties.
IE 完整支持 9 Opera 完整支持 20 Safari 完整支持 7 WebView Android 完整支持 ≤37 Chrome Android 完整支持 27 Firefox Android 完整支持 4 Opera Android 完整支持 14 Safari iOS 完整支持 7 Samsung Internet Android 完整支持 1.5
ex unit Chrome 完整支持 1 Edge 完整支持 12 Firefox 完整支持 1 IE 完整支持 4 Opera 完整支持 3.5 Safari 完整支持 1 WebView Android 完整支持 1 Chrome Android 完整支持 18 Firefox Android 完整支持 4 Opera Android 完整支持 10.1 Safari iOS 完整支持 1 Samsung Internet Android 完整支持 1.0
ic unit Chrome 不支持 No Edge 不支持 No Firefox 不支持 No IE 不支持 No Opera 不支持 No Safari 不支持 No WebView Android 不支持 No Chrome Android 不支持 No Firefox Android 不支持 No Opera Android 不支持 No Safari iOS 不支持 No Samsung Internet Android 不支持 No
lh unit Chrome 不支持 No Edge 不支持 No Firefox 不支持 No IE 不支持 No Opera 不支持 No Safari 不支持 No WebView Android 不支持 No Chrome Android 不支持 No Firefox Android 不支持 No Opera Android 不支持 No Safari iOS 不支持 No Samsung Internet Android 不支持 No
mozmm unit 非标 Chrome 不支持 No Edge 不支持 No Firefox 不支持 4 — 59 IE 不支持 No Opera 不支持 No Safari 不支持 No WebView Android 不支持 No Chrome Android 不支持 No Firefox Android 不支持 4 — 59 Opera Android 不支持 No Safari iOS 不支持 No Samsung Internet Android 不支持 No
rem unit Chrome 完整支持 4 Edge 完整支持 12 Firefox 完整支持 3.6 IE 完整支持 9 Opera 完整支持 11.6 Safari 完整支持 5 WebView Android 完整支持 2 Chrome Android 完整支持 18 Firefox Android 完整支持 4 Opera Android 完整支持 12 Safari iOS 完整支持 4 Samsung Internet Android 完整支持 1.0
rlh unit Chrome 不支持 No Edge 不支持 No Firefox 不支持 No IE 不支持 No Opera 不支持 No Safari 不支持 No WebView Android 不支持 No Chrome Android 不支持 No Firefox Android 不支持 No Opera Android 不支持 No Safari iOS 不支持 No Samsung Internet Android 不支持 No
vb unit Chrome 不支持 No Edge 不支持 No Firefox 不支持 No IE 不支持 No Opera 不支持 No Safari 不支持 No WebView Android 不支持 No Chrome Android 不支持 No Firefox Android 不支持 No Opera Android 不支持 No Safari iOS 不支持 No Samsung Internet Android 不支持 No
vh unit Chrome 完整支持 20 Edge 完整支持 12 Firefox 完整支持 19 注意事项
完整支持 19 注意事项
注意事项 Starting with version 21, viewport-percentage lengths are invalid in @page .
IE 完整支持 9 Opera 完整支持 20 Safari 完整支持 6 WebView Android 完整支持 ≤37 Chrome Android 完整支持 25 Firefox Android 完整支持 19 注意事项
完整支持 19 注意事项
注意事项 Starting with version 21, viewport-percentage lengths are invalid in @page .
Opera Android 完整支持 14 Safari iOS 完整支持 6 Samsung Internet Android 完整支持 1.5
vi unit Chrome 不支持 No Edge 不支持 No Firefox 不支持 No IE 不支持 No Opera 不支持 No Safari 不支持 No WebView Android 不支持 No Chrome Android 不支持 No Firefox Android 不支持 No Opera Android 不支持 No Safari iOS 不支持 No Samsung Internet Android 不支持 No
vmax unit Chrome 完整支持 26 Edge 完整支持 16 Firefox 完整支持 19 注意事项
完整支持 19 注意事项
注意事项 Starting with version 21, viewport-percentage lengths are invalid in @page .
IE 不支持 No Opera 完整支持 15 Safari 完整支持 6.1 WebView Android 完整支持 1.5 Chrome Android 完整支持 26 Firefox Android 完整支持 19 注意事项
完整支持 19 注意事项
注意事项 Starting with version 21, viewport-percentage lengths are invalid in @page .
Opera Android 完整支持 14 Safari iOS 完整支持 6.1 Samsung Internet Android 完整支持 1.5
vmin unit Chrome 完整支持 26 Edge 完整支持 12
完整支持 12
完整支持 12 Alternate Name
Alternate Name Uses the non-standard name: vm
Firefox 完整支持 19 注意事项
完整支持 19 注意事项
注意事项 Starting with version 21, viewport-percentage lengths are invalid in @page .
IE 完整支持 10
完整支持 10
完整支持 9 Alternate Name
Alternate Name Uses the non-standard name: vm
Opera 完整支持 15 Safari 完整支持 6.1 WebView Android 完整支持 ≤37 Chrome Android 完整支持 26 Firefox Android 完整支持 19 注意事项
完整支持 19 注意事项
注意事项 Starting with version 21, viewport-percentage lengths are invalid in @page .
Opera Android 完整支持 14 Safari iOS 完整支持 6.1 Samsung Internet Android 完整支持 1.5
vw unit Chrome 完整支持 20 Edge 完整支持 12 Firefox 完整支持 19 注意事项
完整支持 19 注意事项
注意事项 Starting with version 21, viewport-percentage lengths are invalid in @page .
IE 完整支持 9 Opera 完整支持 20 Safari 完整支持 6 WebView Android 完整支持 ≤37 Chrome Android 完整支持 25 Firefox Android 完整支持 19 注意事项
完整支持 19 注意事项
注意事项 Starting with version 21, viewport-percentage lengths are invalid in @page .
Opera Android 完整支持 14 Safari iOS 完整支持 6 Samsung Internet Android 完整支持 1.5

图例

完整支持

完整支持

不支持

不支持

非标。预期跨浏览器支持较差。

非标。预期跨浏览器支持较差。

见实现注意事项。

见实现注意事项。

使用非标名称。

使用非标名称。

另请参阅

元数据

  • 最后修改：
  1. CSS
  2. CSS 参考
  3. CSS Types
  4. 特性
    1. <angle-percentage>
    2. <angle>
    3. <basic-shape>
    4. <blend-mode>
    5. <custom-ident>
    6. <dimension>
    7. <frequency-percentage>
    8. <frequency>
    9. <gradient>
    10. <image>
    11. <integer>
    12. <length-percentage>
    13. <length>
    14. <number>
    15. <percentage>
    16. <ratio>
    17. <resolution>
    18. <shape>
    19. <string>
    20. <time-percentage>
    21. <time>
    22. <transform-function>
    23. <url>
    24. color
    25. flex
    26. ident
    27. position

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