Why conversion is only approximate
Inch and French sizes are nominal labels, not exact measurements. ETRTO sizes are exact for width and bead seat diameter. So converting from ETRTO to anything else is clean for width but loose for the diameter label, while converting from an inch label to ETRTO needs a lookup, not just arithmetic. Many mistakes come from assuming the diameter in inches multiplies out to a rim size. It does not.
Converting inch widths to millimeters
Width is the easy part. One inch is 25.4 mm.
| Inch width | Approx. millimeters |
|---|---|
| 1.5 | 38 |
| 1.75 | 44 |
| 1.95 | 50 |
| 2.1 | 53 |
| 2.25 | 57 |
| 2.4 | 61 |
| 2.6 | 66 |
Real tires often measure a little different from their label depending on rim width and brand.
Fractional inch sizes
Older sizes use fractions. Converting fractions to millimeters:
- 1 1/8 inch is about 29 mm (often sold as 28 mm)
- 1 1/4 inch is about 32 mm
- 1 3/8 inch is about 35 mm
- 1 1/2 inch is about 38 mm
Diameter: name to ETRTO bead seat diameter
For diameter, use a lookup. Common examples:
| Name | Bead seat diameter |
|---|---|
| 29 inch, 700c, 28 inch (modern) | 622 mm |
| 27.5 inch, 650b | 584 mm |
| 26 inch (mountain) | 559 mm |
| 24 inch | 507 mm |
| 20 inch (BMX) | 406 mm |
More sizes are in the bike tire size chart.
French letters
Older French sizing used letters to indicate rim diameter categories, so 650b and 650c are different sizes: 650b is 584 mm and 650c is 571 mm. For 700c, the letter is associated with 622 mm. Read more in bike tire sizes explained.
Worked examples
26 x 1.95: width about 50 mm and bead seat diameter 559 mm, so roughly 50-559.29 x 2.2: about 56 mm wide on 622 mm, so roughly 56-622.700x28c: 28 mm wide on 622 mm, so 28-622.27.5 x 2.4: about 61 mm wide on 584 mm, so roughly 61-584.
Motorcycle and vehicle sizes are different
Some searches mention sizes like 3.00-21 versus 90/90-21, or 35 inch tires. These are motorcycle or vehicle sizing systems, not bicycle tires, and bike ETRTO numbers do not map onto them. Check the tire maker’s own chart for those, and do not treat them as equivalents to anything on this page.
When a conversion does not mean it fits
Even a correct conversion only tells you the nominal size. You still need to confirm the bead seat diameter matches your rim, the width suits your rim, and your frame clears the tire. If you are thinking of changing wheel size altogether, read bike wheel size conversion first. To measure what you already have, see how to find and measure your bike tire size.