When you need this conversion
The spec is metric, the software is typographic. Packaging, signage and labels are usually specified in millimeters, while stroke weights, font sizes and leading in InDesign and Illustrator are set in points. Convert once and type in the exact value.
Common values
- 0.1 mm = 0.283465 pt (hairline)
- 0.25 mm = 0.708661 pt (thin rule)
- 0.5 mm = 1.41732 pt
- 1 mm = 2.83465 pt
- 2.5 mm = 7.08661 pt
- 4 mm = 11.3386 pt
Leading and grids
Metric baseline grids are common in European editorial design. A 4 mm grid is 11.3386 pt, so body text set “10 on 11.34” aligns perfectly to it. Typing the exact point value avoids the drift you get from rounding to 11 pt over a long column.
Common mistakes
Assuming mm of text height equals mm of font size. Font size measures the em square, not the letters. See points to millimeters for how the two relate.
Rounding stroke weights heavily. 0.25 mm rounded to 0.5 pt is really 0.176 mm, about 30% thinner than specified.