Units in Typography

The two units of measurement most commonly used for typesetting and design are points (pt) and picas (p). While their exact “sizes” have evolved slightly over the centuries, the following explanations clarify the current accepted definitions and representations. It’s simple to convert either measurement to the other unit [source].

Picas

A pica is a hair less than 1/6 inch, and contains 12 points. Picas are typically used to represent fixed horizontal measurements, most often column width. They are commonly used when designing newspapers, magazines, newsletters, and ads. Picas are designated with the letter p, such as 16p.

For instance, the standard width for one column of text on a three-column grid on an 8.5" x 11" document is 14 picas and 4 points, or 14p4 [source].

Another example: 1p6 =  (12 pt + 6 pt = 18 pt) = 0.25 in

Points

A point is a physical unit of length and is equal to 1/72 inch. To be extremely precise, 1 point is equal to .013836 inch, so 72 points are actually .996264 inch. For practical purposes, this is rounded up. Points are the measurement most commonly used in print to indicate the size of type, as well as the space between lines, referred to as line spacing or leading. In some instances, points are also used to measure the width and depth of a column. Points are routinely abbreviated as ‘pt’; typographers and typesetters have traditionally specified a given type setting as 12/16, to indicate 12 point type with 16 point leading [source].

Since 72 points = 1 inch, and if a computer monitor has 72 ppi then 1 point = 1 pixel.

For instance, a point is equal to 1/12 Pica, and 1 Pica = 1/6 inch therefore 1 pt = 1/72 inch.

In CSS

A pt is 1/72 of an in, and a px is 1/96 of an in.

A px is therefore 0.75 pt [source].

In CSS, everything is somewhat abstracted, so a unit such as a "pt" is not necessarily one point in physical size, especially on a screen, an "in" is not necessarily one inch in size, and so forth. Even a "px" is no longer necessarily one pixel in size anymore: Everything is scaled to be consistent with a hypothetical 96 ppi device viewed at normal reading distance, meaning that on screens that differ significantly from 96 ppi or from normal reading distance, everything will be scaled, but still maintain the same relationships ie a pt will still be 1.33334 px units and still be 1/72 of an in unit [source].

An Em Space = size of type
An En Space = half of em space