<length>
<length>
The <length>
CSS data type denotes distance measurements. It is a <number>
immediately followed by a length unit (px
, em
, pc
, in
, mm
, …). Like for any CSS dimension, there is no space between the unit literal and the number. The length unit is optional after the <number>
0
.
Many CSS properties take <length>
values, such as width
, margin
, padding
, font-size
, border-width
, text-shadow
, …
For some properties, using negative lengths is a syntax error, but for some properties, 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.
Interpolation
Values of the <length>
CSS data type can be interpolated in order to allow animations. In that case they 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.
Units
Relative length units
Font-relative lengths
em
- This unit represents the calculated
font-size
of the element. If used on thefont-size
property itself, it represents the inherited font-size of the element.This unit is often used to create scalable layouts, which keep the vertical rhythm of the page, even when the user changes the size of the fonts. The CSS propertiesline-height
,font-size
,margin-bottom
andmargin-top
often have values expressed in em. ex
- This unit 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. ch
- This unit 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
- This unit represents the
font-size
of the root element (e.g. the font-size of the<html>
element). When used on thefont-size
on this root element, it represents its initial value.This unit is practical in creating perfectly scalable layout. 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
line-height
property on the root element, converted to an absolute length. When specified on thefont-size
orline-height
properties of the root element, the rlh units refer to the properties' initial value.
Viewport-percentage lengths
Viewport-percentage lengths defined a length relatively to the size of viewport, that is the visible portion of the document. Only Gecko-based browsers are updating the viewport values dynamically, when the size of the viewport is modified; by modifying the size of the window on a desktop computer, or by turning the device on a phone or a tablet.
In conjunction with overflow:auto
, space taken by eventual scrollbars is not subtracted from the viewport, whereas it is subtracted in the case of overflow:scroll
.
In a @page
at-rule declaration block, the use of the viewport lengths are invalid and the declaration will be dropped.
vh
- 1/100th of the height of the viewport.
vw
- 1/100th of the width of the viewport.
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
- 1/100th of the minimum value of the height and the width of the viewport.
vmax
- 1/100th of the maximum value of the height and the width of the viewport.
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
), 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 purpose. To allow for usable layouts whatever is the used 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 always96px.
-
3pt
is always4px
. -
25.4mm
is always96px.
Specifications
Specification | Status | Comment |
---|---|---|
Unknown The definition of '<length>' in that specification. | Unknown | Added the vi, vb, ic, lh and rlh units |
CSS Values and Units Module Level 3 The definition of '<length>' in that specification. | Candidate Recommendation | Added the ch , rem , vw , vh , vmin , vmax and q units |
CSS Level 2 (Revision 1) The definition of '<length>' in that specification. | Recommendation |
pt , pc , px are explicitly defined (were implicitly defined in CSS1) |
CSS Level 1 The definition of '<length>' in that specification. | Recommendation | Initial definition |
Browser compatibility
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 and rlh | 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 and rlh | 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
.
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Licensed under the Creative Commons Attribution-ShareAlike License v2.5 or later.
https://developer.mozilla.org/en-US/docs/Web/CSS/length