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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
ETRTO (European Tyre and Rim Technical Organisation) is the international standard for tyre sizing. It uses two numbers: width-bead diameter in millimetres (e.g. 25-622 for a 700x25C).
622mm = 700C (road), 584mm = 650B/27.5" (gravel/MTB), 559mm = 26" (MTB), 507mm = 24" (junior).
Multiple naming systems developed independently: French (700C), imperial (27"), and ETRTO. Always use ETRTO for accurate compatibility checking.
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Last updated: June 2026
All calculations are estimates. Always verify results and consult a professional bike fitter where appropriate.