Sources and method
Systems
ISO 8653: size = inner circumference in mm. Germany and the Netherlands: inner diameter. Italy, Spain, Switzerland: circumference − 40. US and Canada: circumference = 2.55 × size + 36.5 mm. UK (BS 6820): C = 40 mm, 1.25 mm per letter. Japan: a jewellers' rule, diameter = 13 + (size − 1) ÷ 3 mm.
On-screen measuring
A bank card is 85.60 × 53.98 mm (ISO/IEC 7810 ID-1). Matching the drawn card to a real one gives the number of screen pixels per millimetre, kept in your browser for next time.
Corrections
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