Definition - Tyre and Rim Sizing

This page lists the different systems of sizing tyres on bicycles. All examples provided use the same sizes where possible. 

French System

The most common sizing system that is used when buying tyres or innertubes is the “French” system. This system is what is normally displayed on the outer rim of tyres, and also on innertube boxes in Europe.

Format example

700x35C

Explanation

The definition of this format is explained below:

Fragment of format 700 x 35 C
General explanation The outer diameter of the tyre in millimeters (mm) - measuredfrom the very bottom of the tyre at its lowest point to the very top at its highest point. Indicator for “dimensions” ("by") The width of the tyre in millimeters (mm) - measured from the cross-section of the tyre from one side to the other (it is not measured over the curve of the outer part of the tyre) The letter code
Technical name Nominal Outside diameter Dimension Indicator Nominal Width Code/Letter code

The letter code is a range of letters from “A” to “D”, which was originally used to refer to the width of the tyre with “A” referring to a narrow tyre, and “D” to a wide tyre. This is roughly true with modern tyres, but “D” is not very often, if ever, used. 

ISO / European Tyre and Rim Technical Organisation (ETRTO) System (Tyre)

The system outlined in ISO 5755-1:2014 is a very similar format to the French format, which measures the height of the wheel and its width. This measurement format specifically excludes the size of the rim of the tyre, which is measured separately 

Format example

37-622

Explanation

The definition of this format is explained below:

Fragment of format 37 - 622
General explanation The measurement of the width of the tire in millimeters (mm), from the widest part of the tire on one side to the widest part of the tire on the other.  Separator The measurement of the inner part of the wheel in millimeters (mm), from the inner part of the wheel rim on one side to the inner part of the wheel rim on the other side.
Technical name Tire section width Separator Bead Diameter

Revision #1
Created Mon, May 3, 2021 12:08 PM by Milleuda
Updated Mon, May 3, 2021 12:08 PM by Milleuda