How to use the minimum readable font size calculator
Exhibition panels and wayfinding signs need larger type as the viewing distance grows. This calculator takes a viewing distance and a minimum viewing angle and returns the physical height the character has to reach, in millimetres, inches and points. Add the height you plan to use and it compares the two and reports the shortfall.
The formula is the angular one: height = 2 x distance x tan(angle / 2), with arc minutes converted at 1/60 of a degree. Points use 1 pt = 1/72 in. The formula and the intermediate values, the degree conversion and the tangent, are printed in the result table so you can check them by hand. Written as of October 2026.
The viewing angle is not supplied by this tool. The arc-minute figures quoted as a legibility minimum differ between sources and none is authoritative. The cap height ratio also differs per font, so it is an input, and the font size is skipped when you leave it blank. Lighting, contrast, stroke weight and differences in eyesight are not modeled, so confirm the result by looking at the real thing from the real distance.
Frequently Asked Questions
The tool does not choose one for you. The arc-minute figures quoted as a legibility minimum differ between sources, so picking one and presenting it as a recommendation would be an invented number. Enter the figure from whichever standard you are using.
The formula returns the physical height the character occupies. Font size in em is larger than that, and the share of the em taken up by the cap height differs per font, so the font size is only worked out when you supply that ratio.
It works from distance and viewing angle to a physical height, which suits printed signs and exhibition panels. Calculations driven by screen diagonal or video resolution are handled by separate tools.