Convert typography

About the units

• Points (pt) – A point is today exactly one seventy-second of an inch, 0.35278 millimetres. The value is fixed in the W3C CSS specification and matches the PostScript point. Type sizes are given in points in word processing, layout and on the web, and the EU energy label prescribes exact point sizes for its text. The older printer’s point was one 72.27th of an inch, and it survives in TeX-set literature – 0.374% smaller for the same figure. A point is also 4/3 of a pixel.
• Pixels (px) – The pixel is an absolute length in CSS: exactly one ninety-sixth of an inch, 0.2646 millimetres and 0.75 points. That is why 16 pixels is exactly 12 points. W3C defines the reference pixel as a visual angle – the size a pixel has on a 96 dpi screen at arm’s length – not as a physical length. And there is the trap: a CSS pixel is not a physical screen pixel. On a high-density display it covers two or three physical pixels.
• Millimetres (mm) – In typography the millimetre is converted into points: 72 divided by 25.4, that is 2.834645669 points per millimetre. The chain runs through the inch, since both the point and the pixel are defined from it. Every print job needs the conversion, with paper sizes, margins and bleed in millimetres while the text is set in points – the EU fuel label is 297 × 210 millimetres, while energy label text is specified in points. The trap is assuming a point is roughly a millimetre.
• Inches (in) – In typography an inch is exactly 72 points, 6 picas and 25.4 millimetres. The W3C CSS specification builds its physical units on it: an inch is 96 pixels, a point one 72nd and a pica one sixth of the inch. The trap is the screen. On paper a CSS inch is a real inch, but screens anchor the units to the pixel, and the specification admits that the physical units may then not match reality, because too much content relied on 96 dpi, 96 pixels to the inch.
• Picas (pc) – A pica is 12 points, one sixth of an inch or 4.233 millimetres. The value is fixed in the W3C CSS specification, where the unit is written pc. The specification also notes that lengths in publishing contexts are sometimes written “2p3”, meaning 2 picas and 3 points. The trap is the older printer’s pica. In TeX a pica is 12 old points of one 72.27th of an inch, that is 4.218 millimetres, 0.37% smaller than in CSS. The same column width therefore gets different figures in the two systems.