Moving between DTP, print and web design means switching units constantly: type is set in points (pt), layouts are checked in millimetres (mm) and websites are built in pixels (px). With the Typography: pt, px, mm & Pica you convert all of these in a single step. This guide explains how the units relate so you can interpret the results correctly.
The common basis: the inch
Every typographic unit is derived from one shared reference – the inch. The fixed relationship is 1 inch = 72 pt = 6 pica = 25.4 mm, which means 1 pica = 12 pt. Points, picas and millimetres are therefore fixed physical measures: a value in pt always has the same physical size, regardless of the device or screen.
Why pixels are a special case
Pixels (px) are not a fixed physical length; they depend on resolution. The rule is px = inch × dpi. The CSS standard assumes 96 dpi, so the browser always works with this reference resolution. If the resolution changes – for example in high-resolution print or other output devices – only the px value changes; pt, mm and pica stay the same.
Conversion example: 12 pt
Take a typical font size of 12 pt at 96 dpi. Via the inch: 12 pt = 12/72 inch = 1/6 inch. At 96 dpi this is exactly 16 px, plus 4.23 mm and 1 pica. This links the classic type size directly to the web value of 16 px, which is also the default font size in most browsers.
Conversion table (at 96 dpi)
| pt | px (96 dpi) | mm | pica |
|---|---|---|---|
| 6 | 8 | 2.12 | 0.5 |
| 9 | 12 | 3.18 | 0.75 |
| 12 | 16 | 4.23 | 1 |
| 18 | 24 | 6.35 | 1.5 |
| 24 | 32 | 8.47 | 2 |
| 36 | 48 | 12.7 | 3 |
The pt, mm and pica columns always apply; only the px column shifts when you assume a resolution other than 96 dpi. For daily work between print and web, open the Typography: pt, px, mm & Pica, set the required resolution and read all four values at once.
