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<length>

The <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, and text-shadow.

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

Syntax

The <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 number and the unit literal. Specifying the length unit is optional if the number is 0.

Note: Some properties allow negative <length> values, while others do not.

The specified value of a length (specified length) is represented by its quantity and unit. The computed value of a length (computed length) is the specified length resolved to an absolute length, and its unit is not distinguished.

The <length> units can be relative or absolute. Relative lengths represent a measurement in terms of some other distance. Depending on the unit, this distance can be the size of a specific character, the line height, or the size of the viewport. Style sheets that use relative length units can more easily scale from one output environment to another.

Note: Child elements do not inherit the relative values as specified for their parent; they inherit the computed values.

The relative length units listed here are based on font and viewport.

Relative length units based on font

Font 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.

Note: 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 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. In 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.

Relative length units based on viewport

The viewport-percentage length units are based on four different viewport sizes: small, large, dynamic, and default. The allowance for the different viewport sizes is in response to browser interfaces expanding and retracting dynamically and hiding and showing the content underneath.

Small

When you want the smallest possible viewport in response to browser interfaces expanding dynamically, you should use the small viewport size. The small viewport size allows the content you design to fill the entire viewport when browser interfaces are expanded. Choosing this size might also possibly leave empty spaces when browser interfaces retract.

For example, an element that is sized using viewport-percentage units based on the small viewport size, the element will fill the screen perfectly without any of its content being obscured when all the dynamic browser interfaces are shown. When those browser interfaces are hidden, however, there might be extra space visible around the element. Therefore, the small viewport-percentage units are "safer" to use in general, but might not produce the most attractive layout after a user starts interacting with the page.

The small viewport size is represented by the sv prefix and results in the sv* viewport-percentage length units. The sizes of the small viewport-percentage units are fixed, and therefore stable, unless the viewport itself is resized.

Large

When you want the largest possible viewport in response to browser interfaces retracting dynamically, you should use the large viewport size. The large viewport size allows the content you design to fill the entire viewport when browser interfaces are retracting. You need to be aware though that the content might get hidden when browser interfaces expand.

For example, on mobile phones where the screen real-estate is at a premium, browsers often hide part or all of the title and address bar after a user starts scrolling the page. When an element is sized using a viewport-percentage unit based on the large viewport size, the content of the element will fill the entire visible page when these browser interfaces are hidden. However, when these retractable browser interfaces are shown, they can hide the content that is sized or positioned using the large viewport-percentage units.

The large viewport unit is represented by the lv prefix and results in the lv* viewport-percentage units. The sizes of the large viewport-percentage units are fixed, and therefore stable, unless the viewport itself is resized.

Dynamic

When you want the viewport to be automatically sized in response to browser interfaces dynamically expanding or retracting, you can use the dynamic viewport size. The dynamic viewport size allows the content you design to fit exactly within the viewport, irrespective of the presence of dynamic browser interfaces.

The dynamic viewport unit is represented by the dv prefix and results in the dv* viewport-percentage units. The sizes of the dynamic viewport-percentage units are not stable, even when the viewport itself is unchanged.

Note: While the dynamic viewport size can give you more control and flexibility, using viewport-percentage units based on the dynamic viewport size can cause the content to resize while a user is scrolling a page. This can lead to degradation of the user interface and cause a performance hit.

Default

The default viewport size is defined by the browser. The behavior of the resulting viewport-percentage unit could be equivalent to the viewport-percentage unit based on the small viewport size, the large viewport size, an intermediate size between the two, or the dynamic viewport size.

Note: For example, a browser might implement the default viewport-percentage unit for height (vh) that is equivalent to the large viewport-percentage height unit (lvh). If so, this could obscure content on a full-page display while the browser interface is expanded.

Viewport-percentage lengths define <length> values in percentage relative to the size of the initial containing block, which in turn is based on either the size of the viewport or the page area, i.e., the visible portion of the document. When the height or width of the initial containing block is changed, the elements that are sized based on them are scaled accordingly. There is a viewport-percentage length unit variant corresponding to each of the viewport sizes, as described below.

Note: Viewport lengths are invalid in @page declaration blocks.

vh

Represents a percentage of the height of the viewport's initial containing block. 1vh is 1% of the viewport height. For example, if the viewport height is 300px, then a value of 70vh on a property will be 210px.

For small, large, and dynamic viewport sizes, the respective viewport-percentage units are svh, lvh, and dvh. vh represents the viewport-percentage length unit based on the browser default viewport size.

vw

Represents a percentage of the width of the viewport's initial containing block. 1vw is 1% of the viewport width. For example, if the viewport width is 800px, then a value of 50vw on a property will be 400px.

For small, large, and dynamic viewport sizes, the respective viewport-percentage units are svw, lvw, and dvw. vw represents the viewport-percentage length unit based on the browser default viewport size.

vmax

Represents in percentage the larger of vw and vh.

For small, large, and dynamic viewport sizes, the respective viewport-percentage units are svmax, lvmax, and dvmax. vmax represents the viewport-percentage length unit based on the browser default viewport size.

vmin

Represents in percentage the smaller of vw and vh.

For small, large, and dynamic viewport sizes, the respective viewport-percentage units are svmin, lvmin, and dvmin. vmin represents the viewport-percentage length unit based on the browser default viewport size.

vb

Represents percentage of the size of the initial containing block, in the direction of the root element's block axis.

For small, large, and dynamic viewport sizes, the respective viewport-percentage units are svb, lvb, and dvb, respectively. vb represents the viewport-percentage length unit based on the browser default viewport size.

vi

Represents a percentage of the size of the initial containing block, in the direction of the root element's inline axis.

For small, large, and dynamic viewport sizes, the respective viewport-percentage units are svi, lvi, and dvi. vi represents the viewport-percentage length unit based on the browser default viewport size.

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 anchoring 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 necessarily 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 1in).

Note: Many users increase their user agent's default font size to make text more legible. Absolute lengths can cause accessibility problems because 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.

Interpolation

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.

Examples

Comparing different length units

The following example 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 the Enter or the Return key.

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

HTML

<div class="outer">
  <div class="input-container">
    <label for="length">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();
})

Result

Specifications

Specification
CSS Values and Units Module Level 5
# lengths

Browser compatibility

Desktop Mobile
Chrome Edge Firefox Internet Explorer Opera Safari WebView Android Chrome Android Firefox for Android Opera Android Safari on IOS Samsung Internet
Q
63
79
49
No
50
No
63
63
49
46
No
8.0
length
1
12
1
3
3.5
1
≤37
18
4
10.1
1
1.0
cap
No
No
97
No
No
No
No
No
97
No
No
No
ch
27
12
1
["From Firefox 1 to Firefox 3, ch was the width of the M character.", "From Firefox 1 to Firefox 3, ch did not work with border-width and outline-width CSS properties."]
9
15
7
4.4
27
4
15
7
1.5
ex
1
12
1
4
3.5
1
4.4
18
4
10.1
1
1.0
ic
No
No
97
No
No
15.4
No
No
97
No
15.4
No
lh
No
No
No
No
No
No
No
No
No
No
No
No
rem
4
12
3.6
9
11.6
5
2
18
4
12
4
1.0
rlh
No
No
No
No
No
No
No
No
No
No
No
No
vb
No
No
101
No
No
15.4
No
No
101
No
15.4
No
vh
20
12
19
Starting with version 21, viewport-percentage lengths are invalid in @page.
9
20
6
4.4
25
19
Starting with version 21, viewport-percentage lengths are invalid in @page.
14
6
1.5
vi
No
No
101
No
No
15.4
No
No
101
No
15.4
No
viewport_percentage_units_dynamic
No
No
101
No
No
15.4
No
No
101
No
15.4
No
viewport_percentage_units_large
No
No
101
No
No
15.4
No
No
101
No
15.4
No
viewport_percentage_units_small
No
No
101
No
No
15.4
No
No
101
No
15.4
No
vmax
26
16
19
Starting with version 21, viewport-percentage lengths are invalid in @page.
No
15
7
1.5
26
19
Starting with version 21, viewport-percentage lengths are invalid in @page.
14
7
1.5
vmin
26
12
12
19
Starting with version 21, viewport-percentage lengths are invalid in @page.
10
9
15
7
4.4
26
19
Starting with version 21, viewport-percentage lengths are invalid in @page.
14
7
1.5
vw
20
12
19
Starting with version 21, viewport-percentage lengths are invalid in @page.
9
20
6
4.4
25
19
Starting with version 21, viewport-percentage lengths are invalid in @page.
14
6
1.5

See also

© 2005–2022 MDN contributors.
Licensed under the Creative Commons Attribution-ShareAlike License v2.5 or later.
https://developer.mozilla.org/en-US/docs/Web/CSS/length