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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 also CSS dimensions, and are usable in some of the same properties that accept <length> values, they are not themselves <length> values.

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.
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 `em`, `pt`, `pc`, and `px` units.
CSS Level 1 The definition of '<length>' in that specification.	Recommendation	Initial definition. Implicit definition of the `em`, `pt`, `pc`, and `px` units.

Browser compatibilityUpdate compatibility data on GitHub

	Desktop
	Chrome	Edge	Firefox	Internet Explorer	Opera	Safari
Basic support	1	Yes	1	3	3.5	1
`cap` unit	No	No	No	No	No	No
`ch` unit	27	12	1 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	20	7
`ex` unit	Yes	Yes	Yes	Yes	Yes	Yes
`ic` unit	No	No	No	No	No	No
`lh` unit	No	No	No	No	No	No
`mozmm` unit	No	No	4 — 59	No	No	No
`Q` unit	63	No	49	No	50	No
`rem` unit	4	12	3.6	9	11.6	4.1
`rlh` unit	No	Yes	No	No	No	No
`vb` unit	No	No	No	No	No	No
`vh` unit	20	12	19	9	20	6
`vi` unit	No	No	No	No	No	No
`vmax` unit	26	?	19	No	20	Yes
`vmin` unit	20	?	19	10 10 9 Uses the non-standard name: `vm`	20	Yes
`vw` unit	20	12	19	9	20	6
Viewport-percentage lengths invalid in `@page`	?	?	21	?	?	?
`1in` is always equal to `96px`	Yes	?	4	No	No	No

	Mobile
	Android webview	Chrome for Android	Edge Mobile	Firefox for Android	Opera for Android	iOS Safari	Samsung Internet
Basic support	Yes	Yes	Yes	4	Yes	Yes	Yes
`cap` unit	No	No	No	No	No	No	No
`ch` unit	Yes	Yes	12	4	Yes	7.1	Yes
`ex` unit	Yes	Yes	Yes	Yes	Yes	Yes	Yes
`ic` unit	No	No	No	No	No	No	No
`lh` unit	No	No	No	No	No	No	No
`mozmm` unit	No	No	No	?	No	No	No
`Q` unit	63	63	No	49	50	No	No
`rem` unit	2	Yes	12	4	12	4	Yes
`rlh` unit	No	No	Yes	No	No	No	No
`vb` unit	No	No	No	No	No	No	No
`vh` unit	Yes	Yes	12	19	Yes	6	Yes
`vi` unit	No	No	No	No	No	No	No
`vmax` unit	1.5	Yes	?	19	No	4	Yes
`vmin` unit	Yes	Yes	?	19	No	6	Yes
`vw` unit	Yes	Yes	12	19	Yes	6	Yes
Viewport-percentage lengths invalid in `@page`	?	?	?	21	?	?	?
`1in` is always equal to `96px`	?	?	?	?	?	?	?

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https://developer.mozilla.org/en-US/docs/Web/CSS/length