<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 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>.

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 unit literal and the number. The length unit is optional after the number 0.

Note: 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.

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 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" (CJK water ideograph, U+6C34) glyph 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 containing block.
vw
Equal to 1% of the width of the viewport's initial containing block.
vi
Equal to 1% of the size of the initial containing block, in the direction of the root element’s inline axis.
vb
Equal to 1% of the size of the initial containing block, 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) doesn'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).

Note: Many users increase their user agent'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.

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.

Specifications

Specification Status Comment
CSS Values and Units Module Level 4
The definition of '<length>' in that specification.
Editor's Draft Adds the vi, vb, ic, lh, and rlh units.
CSS Values and Units Module Level 3
The definition of '<length>' in that specification.
Candidate Recommendation Adds the ch, rem, vw, vh, vmin, vmax, and Q units.
CSS Level 2 (Revision 1)
The definition of '<length>' in that specification.
Recommendation Explicit definition of the empt, pc, and px units.
CSS Level 1
The definition of '<length>' in that specification.
Recommendation Initial definition. Implicit definition of the empt, pc, and px units.

Browser compatibility

Update compatibility data on GitHub
DesktopMobile
ChromeEdgeFirefoxInternet ExplorerOperaSafariAndroid webviewChrome for AndroidFirefox for AndroidOpera for AndroidSafari on iOSSamsung Internet
<length>Chrome Full support 1Edge Full support 12Firefox Full support 1IE Full support 3Opera Full support 3.5Safari Full support 1WebView Android Full support ≤37Chrome Android Full support 18Firefox Android Full support 4Opera Android Full support 10.1Safari iOS Full support 1Samsung Internet Android Full support 1.0
Q unit
Experimental
Chrome Full support 63Edge No support NoFirefox Full support 49IE No support NoOpera Full support 50Safari No support NoWebView Android Full support 63Chrome Android Full support 63Firefox Android Full support 49Opera Android Full support 46Safari iOS No support NoSamsung Internet Android Full support 8.0
cap unit
Experimental
Chrome No support NoEdge No support NoFirefox No support NoIE No support NoOpera No support NoSafari No support NoWebView Android No support NoChrome Android No support NoFirefox Android No support NoOpera Android No support NoSafari iOS No support NoSamsung Internet Android No support No
ch unitChrome Full support 27Edge Full support 12Firefox Full support 1
Notes
Full support 1
Notes
Notes From Firefox 1 to Firefox 3, ch was the width of the M character.
Notes From Firefox 1 to Firefox 3, ch did not work with border-width and outline-width CSS properties.
IE Full support 9Opera Full support 20Safari Full support 7WebView Android Full support ≤37Chrome Android Full support 27Firefox Android Full support 4Opera Android Full support YesSafari iOS Full support 7Samsung Internet Android Full support 1.5
ex unitChrome Full support 1Edge Full support 12Firefox Full support 1IE Full support 4Opera Full support 3.5Safari Full support 1WebView Android Full support 1Chrome Android Full support 18Firefox Android Full support 4Opera Android Full support 10.1Safari iOS Full support 1Samsung Internet Android Full support 1.0
ic unit
Experimental
Chrome No support NoEdge No support NoFirefox No support NoIE No support NoOpera No support NoSafari No support NoWebView Android No support NoChrome Android No support NoFirefox Android No support NoOpera Android No support NoSafari iOS No support NoSamsung Internet Android No support No
lh unit
Experimental
Chrome No support NoEdge No support NoFirefox No support NoIE No support NoOpera No support NoSafari No support NoWebView Android No support NoChrome Android No support NoFirefox Android No support NoOpera Android No support NoSafari iOS No support NoSamsung Internet Android No support No
mozmm unit
ExperimentalNon-standard
Chrome No support NoEdge No support NoFirefox No support 4 — 59IE No support NoOpera No support NoSafari No support NoWebView Android No support NoChrome Android No support NoFirefox Android No support 4 — 59Opera Android No support NoSafari iOS No support NoSamsung Internet Android No support No
rem unitChrome Full support 4Edge Full support 12Firefox Full support 3.6IE Full support 9Opera Full support 11.6Safari Full support 4.1WebView Android Full support 2Chrome Android Full support 18Firefox Android Full support 4Opera Android Full support 12Safari iOS Full support 4Samsung Internet Android Full support 1.0
rlh unit
Experimental
Chrome No support NoEdge No support NoFirefox No support NoIE No support NoOpera No support NoSafari No support NoWebView Android No support NoChrome Android No support NoFirefox Android No support NoOpera Android No support NoSafari iOS No support NoSamsung Internet Android No support No
vb unit
Experimental
Chrome No support NoEdge No support NoFirefox No support NoIE No support NoOpera No support NoSafari No support NoWebView Android No support NoChrome Android No support NoFirefox Android No support NoOpera Android No support NoSafari iOS No support NoSamsung Internet Android No support No
vh unitChrome Full support 20Edge Full support 12Firefox Full support 19
Notes
Full support 19
Notes
Notes Starting with version 21, viewport-percentage lengths are invalid in @page.
IE Full support 9Opera Full support 20Safari Full support 6WebView Android Full support ≤37Chrome Android Full support 25Firefox Android Full support 19
Notes
Full support 19
Notes
Notes Starting with version 21, viewport-percentage lengths are invalid in @page.
Opera Android Full support YesSafari iOS Full support 6Samsung Internet Android Full support 1.5
vi unit
Experimental
Chrome No support NoEdge No support NoFirefox No support NoIE No support NoOpera No support NoSafari No support NoWebView Android No support NoChrome Android No support NoFirefox Android No support NoOpera Android No support NoSafari iOS No support NoSamsung Internet Android No support No
vmax unitChrome Full support 26Edge Full support 16Firefox Full support 19
Notes
Full support 19
Notes
Notes Starting with version 21, viewport-percentage lengths are invalid in @page.
IE No support NoOpera Full support 15Safari Full support 6.1WebView Android Full support 1.5Chrome Android Full support 26Firefox Android Full support 19
Notes
Full support 19
Notes
Notes Starting with version 21, viewport-percentage lengths are invalid in @page.
Opera Android Full support 14Safari iOS Full support 6.1Samsung Internet Android Full support 1.5
vmin unitChrome Full support 26Edge Full support 12
Full support 12
Full support 12
Alternate Name
Alternate Name Uses the non-standard name: vm
Firefox Full support 19
Notes
Full support 19
Notes
Notes Starting with version 21, viewport-percentage lengths are invalid in @page.
IE Full support 10
Full support 10
Full support 9
Alternate Name
Alternate Name Uses the non-standard name: vm
Opera Full support 15Safari Full support 6.1WebView Android Full support ≤37Chrome Android Full support 26Firefox Android Full support 19
Notes
Full support 19
Notes
Notes Starting with version 21, viewport-percentage lengths are invalid in @page.
Opera Android Full support 14Safari iOS Full support 6.1Samsung Internet Android Full support 1.5
vw unitChrome Full support 20Edge Full support 12Firefox Full support 19
Notes
Full support 19
Notes
Notes Starting with version 21, viewport-percentage lengths are invalid in @page.
IE Full support 9Opera Full support 20Safari Full support 6WebView Android Full support ≤37Chrome Android Full support 25Firefox Android Full support 19
Notes
Full support 19
Notes
Notes Starting with version 21, viewport-percentage lengths are invalid in @page.
Opera Android Full support YesSafari iOS Full support 6Samsung Internet Android Full support 1.5

Legend

Full support  
Full support
No support  
No support
Experimental. Expect behavior to change in the future.
Experimental. Expect behavior to change in the future.
Non-standard. Expect poor cross-browser support.
Non-standard. Expect poor cross-browser support.
See implementation notes.
See implementation notes.
Uses a non-standard name.
Uses a non-standard name.

See Also