CSS

Units

CSS Units

CSS has several different units for expressing a length.

Many CSS properties take “length” values, such as width, margin, padding,
font-size, etc.

Length is a number followed by a length unit, such as 10px,
2em, etc.

Example

Set different length values, using px (pixels):

h1 {
  font-size: 60px;
}

p {
  font-size: 25px;
 
line-height: 50px;
}

Try it Yourself »

Note: A whitespace cannot appear between the number and the unit. However, if the value is
0, the unit can be omitted.

For some CSS properties, negative lengths are allowed.

There are two types of length units: absolute and relative.

Absolute Lengths

The absolute length units are fixed and a length expressed in any of these will appear as exactly that size.

Absolute length units are not recommended for use on screen, because screen sizes vary so much.
However, they can be used if the output medium is known, such
as for print layout.

* Pixels (px) are relative to the viewing device. For low-dpi devices, 1px is one device pixel (dot) of the display. For printers and high resolution
screens 1px implies multiple device pixels.

Relative Lengths

Relative length units specify a length relative to another length property.
Relative length units scale better between different rendering medium.

Tip: The em and rem units are practical in creating perfectly
scalable layout!
* Viewport = the browser window size. If the viewport is 50cm
wide, 1vw = 0.5cm.

Browser Support

The numbers in the table specify the first browser version that fully supports the
length unit.

Length Unit

em, ex, %, px, cm, mm, in, pt, pc
1.0
3.0
1.0
1.0
3.5

ch
27.0
9.0
1.0
7.0
20.0

rem
4.0
9.0
3.6
4.1
11.6

vh, vw
20.0
9.0
19.0
6.0
20.0

vmin
20.0
12.0
19.0
6.0
20.0

vmax
26.0
16.0
19.0
7.0
20.0