How to use the QR Frame Text Size Calculator
How this calculator works
Readable text needs a minimum angular size at the eye: as the viewer moves further away, the letters must grow to subtend the same angle. This tool applies a 10-arcminute rule for cap height, then converts cap height to a font size using the usual approximation that cap height is about 0.7 of the font size. The condition factor adds margin for outdoor light, low light, and viewers who are moving.
Why the frame text matters
The call to action is what turns a printed square into a scan: it tells people what they get and what to do. Text below the legible minimum might as well not be there, because viewers will not slow down to decode it. Short text in large type — a few words — beats a sentence nobody can read.
Checking the result in the real world
Print the frame at final size, stand at the real viewing distance, and read it without squinting or stepping closer. If you cannot read it instantly, go bigger. Test in the real light too: bright sun, dim corridors, and glare all shrink effective legibility below what the numbers suggest.