Bicycle Tyre Size Converter

In short Bicycle tyre sizing is a mess of historical names, and ETRTO is the one part of it that is unambiguous. An ETRTO marking like 25-622 means a 25 mm tyre on a rim whose BEAD SEAT diameter is 622 mm, and bead seat diameter is the only measurement that decides whether a tyre will fit a rim. Everything else on the sidewall is a label. The labels collide badly. 700c and 29 inch are the SAME rim — both 622 mm — so a 29er tyre and a 700c tyre mount on identical rims and differ only in the widths people actually make. Meanwhile two completely different rims are both sold as 20 inch, 406 and 451, and they do not interchange at all; the same is true of 349 and 305 both being called 16 inch. And the inch numbers are not measurements: a 26 inch rim has a 559 mm bead seat, which is 22 inches, not 26. The name dates from when tyres were much fatter and the OUTSIDE diameter was near 26 inches. If you are buying a tyre, ignore the inch name and match the ETRTO number. If you are calibrating a computer, note that the circumference here is geometric — the rolling circumference under load is 5 to 15 mm smaller.