Bicycle Tire Size Decoder
Read bike tire codes such as 700x25C. Use the calculator below. Results update as you type.
How the Bicycle Tire Size Decoder Works
This decoder works by reading any of the 3 bicycle size systems (ISO/ETRTO, inch and French) and returning the bead seat diameter (BSD) that decides fit. The BSD is the number that must match the rim.
- Enter the marking, for example 700x28c, 28-622 or 28 x 1-1/8.
- Read the BSD: 622 mm.
- Read the equivalent names in the other systems.
- Use the BSD to confirm the tire fits your rim.
The Formula
The formula is a lookup that maps every marking to its ISO BSD: Fit = (Tire BSD = Rim BSD).
ISO Size = Width (mm) − BSD (mm) Fit = BSD_tire = BSD_rim Outer Diameter ≈ BSD + 2 × WidthWorked example: 27.5 x 2.25 maps to ISO 57-584. A 650B rim has a 584 mm BSD, so the tire fits.
Conversion Table
The conversion table maps 10 bead seat diameters to ISO, inch and French markings and common use. BSDs run from 406 mm (20 in BMX) to 630 mm (27 in road).
The 622 mm row covers road bikes, gravel bikes and 29 in mountain bikes. They all share one BSD.
| Bead Seat Diameter (mm) | ISO Example | Inch Marking | French Marking | Common Use |
|---|---|---|---|---|
| 622 | 28-622 | 28 x 1-5/8 x 1-1/8 / 29 x 2.2 | 700C | Road, gravel, 29er MTB |
| 630 | 32-630 | 27 x 1-1/4 | — | Older road bikes |
| 597 | 32-597 | 26 x 1-1/4 | — | Older British club bikes |
| 590 | 37-590 | 26 x 1-3/8 | 650A | English 3-speeds |
| 584 | 57-584 | 27.5 x 2.25 / 26 x 1-1/2 | 650B | Trail MTB, randonneur |
| 571 | 23-571 | 26 x 1 | 650C | Triathlon, small road |
| 559 | 54-559 | 26 x 2.1 | — | 26 in MTB, fatbike |
| 507 | 50-507 | 24 x 1.95 | — | Junior MTB |
| 451 | 28-451 | 20 x 1-1/8 | — | Recumbent, folding |
| 406 | 50-406 | 20 x 1.95 | — | BMX, folding |
What Is the Bicycle Tire Size Decoder?
The Bicycle Tire Size Decoder is a tool that interprets ISO/ETRTO, inch and French bicycle tire markings. ISO 5775, the international standard, uses the format width-BSD, such as 37-622.
Inch and French names are older and often ambiguous: "26 inch" covers 5 different BSDs. The ISO number removes that ambiguity.
Common Uses
This decoder has 4 uses: replacements, rim matching, vintage bikes and gravel conversions.
- Replace tires: Find the ISO size of the old tire.
- Match rims: Confirm BSD before buying.
- Restore vintage bikes: Decode old inch markings.
- Convert to gravel: Find wider tires on a 622 rim.