Learn About Tyre Sizes

International standards have had a positive impact on the cycling industry. This article aims to explain modern tyre sizes, how they're designated, and how to convert from older systems.

What is ISO 5775?

ISO 5775 is the international standard for labelling the size of bicycle tyres and rims. It was originally developed by the European Tyre and Rim Technical Organisation (ETRTO); and has since been licensed to the International Organization for Standardization (ISO).

How are tyre sizes designated?

This system replaces overlapping informal systems that ambiguously distinguished between sizes. Previously, nominal dimensions were used which were interpreted in different ways by different countries and manufacturers – causing a problem for the end-user.

Tyre markings according to this system are designated with the following format, where TT stands for the thickness of the inflated tyre minus any tread, and TD for the inner diameter of the tyre when it's mounted on the rim.

TT - TD

Where can measurements be found?

The thickness of the tyre is measured across the tread, while the diameter of the rim is measured up to the rim bead. Both are measured in millimetres. A marking can also be found printed on the sidewalls of the tyre.

Which other tyre sizing systems exist?

There exists several other systems, but none are international standards. To learn more about these sizing systems, please visit our articles covering each topic individually.

Why does Derailleur not provide all tyre sizing systems?

Derailleur tries to provide a quality product that all cyclists can trust. We recognize that standards ensure a reliable basis for everyone to share the same expectations about how a product is intended to function; and so we make it our responsibility to follow international standards for everyone's benefit.

How to convert older tyre markings?

Older tyre markings can be converted to ISO 5775 designations with the help of the following conversion table.