CSS has several different units for expressing a length. Many CSS properties take "length" values, such as width, margin, padding, font-size, border-width, etc. Length is a number followed by a length unit, such as 10px, 2em, etc. 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.
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.
cm Centimetersmm Millimetersin Inches (1in = 96px = 2.54cm)pt Points (1pt = 1/72 of 1in)pc Picas (1pc = 12 pt)px pixels (1px = 1/96th of 1in)Relative length units specify a length relative to another length property. Relative length units scales better between different rendering mediums.
em Relative to the font-size of the element (2em means 2 times the size of the current font)ex Relative to the x-height of the current font (rarely used)rem Relative to font-size of the root elementvh Relative to 1% of the height of the viewport% Relative to the parent element