Wheel Sizes
Cycling · 10 specs
Direct Answer
Bicycle tire and rim dimensions are standardized internationally under ISO 5775 and the ETRTO (European Tyre and Rim Technical Organisation) system. Sizing is defined by two numbers: nominal tire width and Bead Seat Diameter (BSD) in millimeters (e.g. 28-622). Major standards include 700c / 29er (BSD 622 mm), 27.5" / 650b (BSD 584 mm), 26" MTB (BSD 559 mm), 650c (BSD 571 mm), and 20" BMX (BSD 406 mm).
Source: ISO 5775 (Bicycle tyres and rims) / ETRTO Standards Manual · Last reviewed Aug 28, 2026
Specs
| 700c Road / 29" MTB BSD | 622.0 mm Bead Seat Diameter (ISO xx-622) |
| 27.5" MTB / 650b Gravel BSD | 584.0 mm Bead Seat Diameter (ISO xx-584) |
| 26" Mountain Bike BSD | 559.0 mm Bead Seat Diameter (ISO xx-559 classic MTB) |
| 650c Triathlon / Road BSD | 571.0 mm Bead Seat Diameter (ISO xx-571) |
| 20" BMX (Standard) BSD | 406.0 mm Bead Seat Diameter (ISO xx-406) |
| 20" BMX (Race / Junior) BSD | 451.0 mm Bead Seat Diameter (ISO xx-451) |
| 16" Folding Bike (Brompton) BSD | 349.0 mm Bead Seat Diameter (ISO xx-349) |
| Road Tire Width Range | 23.0–32.0 mm (e.g. 25-622, 28-622, 32-622) |
| Gravel Tire Width Range | 35.0–50.0 mm (e.g. 40-622, 47-584) |
| MTB Tire Width Range | 53.0–66.0 mm (2.1–2.6 in / e.g. 60-622, 65-584) |
The ISO 5775 / ETRTO Standardization System
- Bead Seat Diameter (BSD) Foundation: Traditional fractional and French tire sizing (e.g. 26 × 1¾" vs. 26 × 1.75") caused dangerous interchangeability errors because their rim diameters were physically incompatible. The ETRTO/ISO standard defines tires strictly by 'Width-BSD' in millimeters (e.g., 28-622 is a 28 mm wide tire on a 622 mm rim).
- 700c and 29er Identity: 700c road wheels and 29-inch mountain bike wheels share the exact same rim Bead Seat Diameter of 622 mm; a '29er' is simply a 622 mm rim fitted with high-volume (2.0–2.5 in) knobby off-road tires.