Relative and absolute length units

Measurements and positions in CSS properties are indicated in length units. Relative units specify a length in relation to another property, and scale better from one output device to another (such as from a monitor to a printer) and in comparison to other page elements. Absolute units specify an exact measurement, such as inches or centimeters, and are useful when you know the physical properties of the output device.

Relative length units Description
% A fluid measurement based on the percentage of space within a given container
em A fluid measurement of space in relation to the computed font-size of the parent container
rem Font size of the root element (not dependent on the font-size of it's parent container)
vw The viewport width (equal to 1% of the width of the initial containing block)
vh The viewport height (equal to 1% of the height of the initial containing block)
vmin The smaller (minimum) value of viewport width or height
vmax The larger (maximum) value of viewport width or height
ex The height of a lowercase "x" in a font.
ch Zero-width, width of the zero glyph in the rendering font (Unicode character U+0030)

In a @page at-rule declaration block, the use of the viewport lengths are invalid and the declaration will be dropped.

Absolute length units Description
px Pixels, relative to the viewing device, a non-fluid unit of measurement (1/96th of a physical inch)
in Inches (1in = 2.54cm = 96px)
cm Centimeters (96px)
mm Millimeters (1/10th of 1cm)
pt Points (1pt = 1/72 inches)
pc Picas (1pc = 12pt)

Relative and absolute font sizes

Setting the font size may change the size of other items, since it is used to compute values such as em and ex length units.

Absolute font sizes Keyword
font-size xx-small;
font-size x-small;
font-size small;
font-size medium;
font-size large;
font-size x-large;
font-size xx-large;
Relative font sizes Keyword
font-size larger;
font-size smaller;
Global font sizes Keyword
font-size inherit;
font-size initial;
font-size unset;