Convert millimeters to points

A millimeter is 2.8346 points, so you get points by multiplying millimeters by 2.8346. Three millimeters come to 8.5 pt, and 25.4 mm, one inch, to exactly 72 pt.

Formula: points = millimeters × 72 ÷ 25.4

The 72 comes from the point being 1/72 inch, and the 25.4 from the length of the inch in millimeters. The CSS standard defines both[1]. Results are rarely round, because the point was built on the inch and the millimeter on the meter. Whole inches and simple fractions of an inch come out round: 50.8 mm, two inches, is 144 pt, and 12.7 mm, half an inch, is 36 pt.

The pt to mm converter handles the other direction and also shows how tall letters are at common font sizes.

Print shops outside the US, sign makers, engravers and technical drawings give sizes in millimeters. The programs you design in set font size, stroke weight and line spacing in points. When a brief says “letters 5 mm tall” or “0.5 mm rule”, the value has to become points before you can type it into a field.

Today's point is 0.3528 mm, the desktop publishing point defined as 1/72 inch[2]. Older European type used the larger Didot point, and TeX still uses a slightly smaller one, which is why the converter has a Point type selector. The default, PostScript (DTP), matches Word, InDesign and web browsers.

If your size is in pixels rather than millimeters, the px to pt converter turns it into points and the px to mm converter into millimeters.

A font size in points measures the body of the type, not the letter, and in digital type each designer decides how tall the letters are within it[3]. So when an engraver or a sign spec asks for letters a set number of millimeters tall, converting those millimeters straight to points gives lettering that is too small.

The capital height is stored in the font file as a share of the font size[4]. We read it from the files that ship with Windows 11: capitals take up 71.6% of the font size in Arial, 66.2% in Times New Roman and 63.2% in Calibri. To pick a size, divide the letter height you need by that share and convert the result to points. A capital 5 mm tall needs a 6.98 mm body in Arial, which is 19.8 pt.

Technical drawings give line widths in millimeters, and a full set of technical pens covers the widths defined in ISO 128[5]. When you redraw such a drawing in a graphics program, the stroke weight has to go in as points.

InDesign accepts a value in a different unit from the one set in its preferences. Type the number with its abbreviation, for example 48 mm, and the program converts it to the unit of the field[8]. The same help page states that an inch holds 72 PostScript points or 72.27 traditional printer's points, and lets you choose between the two in the preferences[8].

The second value is the point behind the Printer's (TeX) option in the selector. TeX took it from the old American point system[2]. If you set a length in a LaTeX document and give it in pt, pick that option, because 10 mm then comes to 28.45 pt rather than 28.35 pt.