<length>

The <length> CSS data type represents a distance value. Many CSS properties take <length> values, such as width, margin, padding, font-size, border-width, text-shadow, and many others.

For some properties a negative length is a syntax error, while for others negative lengths are allowed. Please note that although <percentage> values are also CSS dimensions, and are accepted by some CSS properties that accept <length> values, they are not themselves <length> values.

Syntax

A length consists of a <number> followed by a unit (px, em, pc, in, mm, …). 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.

Interpolation

In animations, length values are interpolated as real, floating-point numbers. The interpolation happens on the calculated value. Its speed is determined by the timing function associated with the animation.

Units

Relative length units

Font-relative lengths

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.
This unit is often used to create scalable layouts, which maintain the vertical rhythm of the page even when the user changes the font size. The CSS properties line-height, font-size, margin-bottom, and margin-top often have values expressed in em.
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.
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.
ic: Equal to the used advance measure of the “水” (CJK water ideograph, U+6C34) glyph found in the font used to render it.
rem: Represents the font-size of the root element (e.g., the font-size of the <html> element). When used within the root elements font-size, it represents its initial value (common browser default is 16px, but changes based upon users preferences).
This unit is often used to create scalable layouts. If not supported by the targeted browsers, such layout can be achieved using the em unit, though this is slightly more complex.
lh: Equal to the computed value of the line-height property of the element on which it is used, converted to an absolute length.
rlh: Equal to the computed value of the line-height property on the root element, converted to an absolute length. When specified on the font-size or line-height properties of the root element, the rlh unit refers to the properties' initial value.

Viewport-percentage lengths

Viewport-percentage lengths define a length relative to the size of the viewport, i.e., the visible portion of the document.

If the html and body are set as overflow:auto, space taken by scrollbars is not subtracted from the viewport, whereas it will be subtracted if set as overflow:scroll.

Viewport lengths are invalid in @page declaration blocks.

vh: Equal to 1% of the height of viewports initial containing block.
vw: Equal to 1% of the width of viewports 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 or vh.
vmax: Equal to the larger of vw or 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, and high-resolution devices, such as printers.

For low-dpi devices, the unit px represents the physical reference pixel and then 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, length described in inches (in), centimeters (cm), or millimeters (mm) doesn't necessary match the length of the physical unit with the same name.

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

Users may increase font size for accessibility purposes. To allow for usable layouts regardless of font size, use only absolute length units when the physical characteristics of the output medium are known, such as bitmap images. When setting length related to font-size, prefer relative units like em or rem.

px: Relative to the viewing device.
For screen display, typically one device pixel (dot) of the display.
For printers and very high resolution screens one CSS pixel implies multiple device pixels, so that the number of pixel per inch stays around 96.
mm: One millimeter.
q: A quarter of a millimeter (1/40th of a centimeter).
cm: One centimeter (10 millimeters).
in: One inch (2.54 centimeters).
pt: One point (1/72nd of an inch).
pc: One pica (12 points).
mozmm: An experimental unit which 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.

CSS units and dots-per-inch

The unit in doesn't represent a physical inch on screen, but represents 96px. That means that whatever is the real screen pixel density, it is assumed to be 96dpi. On devices with a greater pixel density, 1in will be smaller than 1 physical inch. Similarly mm, cm, and pt are not absolute length.

Some specific examples:

1in is always 96px.
3pt is always 4px.
25.4mm is always 96px.

Specifications

Specification	Status	Comment
Unknown The definition of '<length>' in that specification.	Unknown	Addition of the `vi`, `vb`, `ic`, `lh`, and `rlh` units.
CSS Values and Units Module Level 3 The definition of '<length>' in that specification.	Candidate Recommendation	Addition of 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 `pt`, `pc`, and `px` units.
CSS Level 1 The definition of '<length>' in that specification.	Recommendation	Initial definition. Implicit definition of the `pt`, `pc`, and `px` units.

Feature	Chrome	Firefox (Gecko)	Internet Explorer	Opera	Safari
Basic support	1	1.0 (1.7 or earlier)	3.0	3.5	1.0
`ch`	27	1.0 (1.7 or earlier)^[1]	9.0	20.0	7.0
ex	(Yes)	(Yes)	(Yes)	(Yes)	(Yes)
`rem`	4 (532.3)	3.6 (1.9.2)	9.0	11.6	4.1
`vh, vw`	20	19 (19)	9.0	20.0	6.0
`vmin`	20	19 (19)	9.0^[2]	20.0	6.0
`vmax`	26	19 (19)	No support	20.0	(Yes)
Viewport-percentage lengths invalid in `@page`	?	21 (21)	?	?	?
`mozmm`	No support	4.0 (2.0)	No support	No support	No support
`1in` always is `96dpi`	(Yes)	4.0 (2.0)	(Yes)	(Yes)	(Yes)
`q`	No support	49.0 (49.0)	No support	No support	No support
vi, vb, ic, lh, rlh, and cap	No support	No support	No support	No support	No support

Feature	Android	Firefox Mobile (Gecko)	IE Phone	Opera Mobile	Safari Mobile
Basic support	(Yes)	(Yes)	(Yes)	(Yes)	(Yes)
`ch`	No support	(Yes)	7.8	?	7.1.1
ex	?	(Yes)	?	?	?
`rem`	2.1	(Yes)	?	12.0	4.0
`vh, vw, vmin`	(Yes)	19.0 (19)	?	No support	6.0
`vmax`	1.5	19.0 (19)	?	No support	4.0
Viewport-percentage lengths invalid in `@page`	?	21.0 (21.0)	?	?	?
`q`	?	49.0 (49.0)	?	?	No support
vi, vb, ic, lh, rlh, and cap	No support	No support	No support	No support	No support

[1] In Gecko 1.0-1.9.0 (Firefox 1.0-3.0) ch was the width of 'M' and it didn't work for border-width and outline-width CSS properties.

[2] Internet Explorer implements this with the non-standard name vm.