Tyre Size Converter — ETRTO to French/Imperial
Convert between ETRTO, French and imperial tyre size designations.
How We Calculate This
Size Conversion
Outer Diameter = Bead Diameter + (2 × Tyre Width)
Geometric Circumference = Outer Diameter × π
ETRTO is the definitive size standard. The French name (e.g. 700x25C) maps to ETRTO based on bead diameter: 622mm = 700C, 584mm = 650B/27.5″, 559mm = 26″, 507mm = 24″.
The geometric circumference assumes a perfect circle at the outer diameter. A mounted, loaded tyre rolls about 1% smaller, so for cyclecomputer calibration use the real rolling circumference — roughly 2105 mm for a 700x25C versus the 2111 mm geometric figure (Sheldon Brown cyclecomputer-calibration / ETRTO rolling-length charts).
Frequently Asked Questions
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Last updated: 2025-02-20
All calculations are estimates. Always verify results and consult a professional bike fitter where appropriate.