Wheels and tyre sizes: reading the numbers, and the racing rules.

ISO 5775: the millimetre system

Bicycle tyre and rim sizes are set by the international standard ISO 5775, in two parts. BSI lists both as current UK adoptions:

• ISO 5775-1:2023, tyre designations and dimensions, published by BSI on 31 August 2023. It covers clincher tyres on straight-side or crotchet rims, tubeless, tubeless-ready and tube-type tyres, and beaded-edge tyres on hooked bead rims. It does not apply to tubular (sew-up) tyres or to non-pneumatic tyres.
• ISO 5775-2:2021, rims, fourth edition, published 31 March 2021 and marked as under review. It covers the tyre-fitting dimensions and designations of straight-side rims (SS, and TSS for tubeless straight-side), hooked bead rims (HB) and crotchet rims (C, and TC for tubeless crotchet), plus rim contour, valve hole and rim base protection.

Wikipedia says the system was developed by the European Tyre and Rim Technical Organisation (ETRTO). A tyre is labelled by its width and its bead seat diameter in millimetres: in Wikipedia's example, "32-597" is a tyre 32 mm wide that fits a rim with a 597 mm bead seat diameter.

What the older names mean in millimetres

Wikipedia's conversion table matches the common older names to bead seat diameters:

As the table shows, 27-inch and 700C are different sizes despite the similar-sounding names. The gear calculator uses tyre size to work out how far one turn of the cranks takes the bike.

Tyre and rim types

Clincher

Named in ISO 5775-1, which covers clinchers on straight-side or crotchet rims.

Tubeless and tubeless-ready

Also named in ISO 5775-1. Wikipedia says most gravel bikes use tubeless tyres because they resist punctures and pinch flats better than clinchers with tubes.

Tubular (sew-up)

Excluded from ISO 5775-1's scope.

Hooked and hookless rims

ISO 5775-2 distinguishes hooked bead and crotchet rims from straight-side rims. Schwalbe, quoted below, uses "hookless" for straight-side rims.

Hookless rims and pressure: one maker's statement

Schwalbe, a tyre maker, says that under ETRTO rules hookless (straight-side) road rims have a maximum pressure of 5 bar, and that where a rim maker sets a lower limit, that limit takes precedence over a higher figure printed on the tyre. The same Schwalbe page also gives a width-based ETRTO table allowing 5.5 bar for 18 to 24 mm tyres on hookless rims, which does not match its single 5 bar figure. We could not open ETRTO's own document, so the correct figure is unconfirmed. Schwalbe also says only its tyres marked TLE or TLR may be used on hookless rims, and that rim makers may set minimum tyre widths, often 28 mm. These are one maker's statements about its own products; the rim and tyre makers' instructions and a qualified bike mechanic are the authority for any particular wheel.

Racing wheels: UCI Article 1.3.018

As reproduced in the Japan Cycling Federation's copy of the UCI clarification guide (uploaded March 2026; edition date unconfirmed; the article lists modification dates up to 1 January 2026):

• Wheels may be 550 to 700 mm in diameter, including the tyre.
• Rims must comply with ISO 5775-2 and tyres with ISO 5775-1.
• In road, track and cyclo-cross only UCI-approved wheel designs may be used. Wheels must pass the impact test of ISO 4210-2:2023 (section 4.10.7.2.2, paragraph 2), with makers applying by declaration of conformity. Wheels on the approved list under the procedure in force until 31 December 2015 remain allowed.
• Wheels for mass-start road and cyclo-cross may have a rim no deeper than 65 mm and must have at least 12 spokes, none with a section dimension over 10 mm. The UCI set the 65 mm limit for road from 1 January 2026.
• "Traditional wheels" need no approval: alloy rims under 25 mm deep, at least 20 detachable steel spokes, and all parts identifiable and on sale.
• Cyclo-cross tyres may be at most 33 mm wide and may not have spikes or studs.
• On the track, a front disc wheel is allowed only in events against the clock; in the individual sprint only for the 200 m qualifying ride.

Wikipedia says time trial bikes may use solid disc wheels or spoked wheels. Mountain bikes may use wheels of different diameters under the UCI's mountain bike exception to Article 1.3.007; we did not read the mountain bike, BMX or trials parts of the UCI regulations, so their wheel-size rules are not given here, except that the UCI groups trials riders by wheel size (20-inch, 26-inch and Open). How the 2026 rim limit was set is on the 2026 changes page.