Before you startGeneral guidance from our members' experience, checked against the sources listed at the foot of this page. Old bicycles vary, so check everything against your own bike. Brakes, steering, wheels and tyres are safety-critical: if you're not sure, have a qualified mechanic do or check the work. Any work you do is at your own risk. See our terms.
Fitting the wrong tyre size is one of the commonest mistakes in restoration. Inch sizes such as '26 × 1⅜' can describe more than one rim. The ISO size (also called ETRTO), written like 37-590, is unambiguous: the second number is the rim's bead seat diameter in millimetres, and that's what has to match.
| Inch size | ISO bead seat (mm) | Typically found on (Sheldon Brown) |
|---|---|---|
| 28 × 1½ | 635 | Traditional roadsters; Raleigh's catalogues give 28 × 1½in for the Tourist |
| 26 × 1⅜ (E.A.3) | 590 | Most English 3-speeds |
| 26 × 1¼; 26 × 1⅜ (S-6) | 597 | Older British sport and club bikes |
| 27 × 1¼ | 630 | Older road bikes. Not the same as 700c |
| 700c | 622 | Road bikes and hybrids |
| 24 × 1⅜ (E.5) | 540 | British juvenile |
| 24 × 1¼; 24 × 1⅜ (S-5) | 547 | British or Schwinn juvenile |
| 20 × 1⅛, 1¼ or 1⅜ | 451 | Juvenile lightweights, some folding bicycles |
| 26 × 1.00 and wider (decimal sizes) | 559 | Most mountain bikes and cruisers |
| 24 × 1.5 to 24 × 2.125 | 507 | Juvenile mountain bikes and cruisers |
| 20 × 1.5 to 20 × 2.125 | 406 | Most BMX, juvenile and folding bicycles |
| 17 × 1¼ | 369 | Alex Moulton AM series |
| 16 × 1⅜ | 349 | Older Moultons; Brompton and other folders |
Find your size
- Read the ISO numberLook on the old tyre's sidewall or on the rim for a number such as 37-590.
- No number? MeasureMeasure the rim's bead seat diameter. Sheldon Brown has a page on measuring rims.
- Order by the ISO numberNot by the inch size. 26 × 1⅜ alone could be 590 or 597.
Reading the sidewall
This BMX tyre is marked 20x2.125. Sheldon Brown's table puts 20 × 1.5 to 20 × 2.125 tyres at ISO 406.
Photos: Garry Gregory.
Never guessA tyre for the wrong bead seat won't seat properly and can come off. A tyre that's too wide can foul the frame or brakes.
Examples of tyres still made in old sizes
- Schwalbe Delta Cruiser in 26 × 1⅜ (37-590), for roadsters and 3-speeds
- Panaracer Pasela in 27 × 1¼ (32-630), for older road bikes
Sources
Checked 19 October 2026. Method tips without a source are our members' experience.