How to use the converter
- Type the size as it appears on the sidewall or in the store listing: “700x28c”, “28-622”, “29x2.4”, “27.5 x 2.25”, “650b x 47”, “26 x 1.95”, “28 x 1 5/8 x 1 3/8”… It doesn’t matter whether you use “x”, ”×”, commas or periods.
- Read the result: the ETRTO size, the rim diameter (BSD), its other names, the outer diameter and the circumference.
- To check whether it fits your rim, add the rim’s inner width. You’ll find it in the wheel specs or printed on the rim (“622x21C” or “21C” means 21 mm).
- Copy the circumference into your bike computer or speed sensor.
If your size only gives the diameter (“700c”, “650b”, “29”), we’ll ask for the width to calculate the circumference.
The three tire sizing systems
The same tire can be named three ways. The only one that never misleads is ETRTO. Equivalents for the best-selling sizes are in the tire size chart.
- ETRTO (ISO 5775): “37-622”. First the tire width in mm, then the BSD (bead seat diameter): the diameter of the rim where the tire bead sits. Two tires with the same BSD fit the same rim, whatever they’re called.
- French: “700 × 35C”. The first number is the approximate outer diameter in mm, the second the width, and the letter (A, B or C) indicates the rim diameter. 700C is 622; 650B is 584; 650C is 571; 650A is 590.
- Inches: “29 × 2.4” or “26 × 1 3/8”. Careful: decimal and fractional inch sizes are not compatible even when they start with the same number. A 26 × 1.95 (559) won’t fit the rim of a 26 × 1 3/8 (590).
Rim diameters (BSD)
| BSD | Names | Use |
|---|---|---|
| 622 | 700C, 29”, 28” | Road, gravel, 29er mountain, city and touring |
| 584 | 650B, 27.5”, 26 × 1 1/2 | 27.5 mountain, 650b gravel |
| 571 | 650C, 26 × 1 3/4 | Triathlon and small road frames |
| 559 | 26” (decimal) | Classic mountain, fat bike, touring |
| 590 | 650A, 26 × 1 3/8 | Classic European city bikes |
| 630 | 27 × 1 1/4 | Road bikes before the 1990s |
| 507 | 24” | Kids and youth |
| 406 | 20” | Kids, BMX and folding bikes |
| 349 | 16 × 1 3/8 | Brompton-style folding bikes |
| 305 | 16” (decimal) | Kids |
| 203 | 12 1/2” | Kids and balance bikes |
Widths in inches and millimeters
Inch widths don’t convert to millimeters by multiplying by 25.4: each size is rounded to an ETRTO width. The equivalents Schwalbe uses:
| Inches | ETRTO | Inches | ETRTO |
|---|---|---|---|
| 1.35 | 35 | 2.10 | 54 |
| 1.40 | 37 | 2.25 | 57 |
| 1.50 | 40 | 2.35 | 60 |
| 1.60 | 42 | 2.40 | 62 |
| 1.75 | 47 | 2.60 | 65 |
| 2.00 | 50 | 2.80 | 70 |
Some sizes are sold with two widths. Schwalbe, for example, sells 26 × 1.95 tires as 50-559 and also as 54-559. When in doubt, the ETRTO number on the sidewall wins.
How we calculate wheel circumference
A mounted, inflated tire isn’t as tall as it is wide: it ends up at about 97% of its nominal width. So:
Outer diameter = BSD + 2 × 0.97 × width
Circumference = π × outer diameter
For a 700×28C: π × (622 + 2 × 0.97 × 28) = 2,125 mm. The CatEye chart gives 2,136 mm. For common sizes, our formula stays within 1% of that chart, and we show both values when your size is listed in it.
Actual circumference depends on the brand, the rim, the pressure and your weight. For the exact value, do a rollout test, which is what Schwalbe recommends:
- Inflate to your usual pressure and put the valve at the bottom. Mark the ground right below it.
- Sit on the bike and roll straight ahead until the valve is back at the bottom.
- Mark again and measure the distance in millimeters. That’s your circumference.
A 10 mm error in circumference means about half a mile of error every 100 miles (0.5 km per 100 km). If you can’t do the test, look up your size in the bike computer wheel circumference chart.
What tire fits your rim
The ETRTO standard sets which tire widths each rim inner width accepts. The 2024 version opened up the ranges a lot compared with the old one, because rims have gotten wider. The full table is in the calculator; some examples:
- 17 mm inner width (older road rims): 20 to 57 mm.
- 21 mm (current road and gravel): 25 to 65 mm.
- 25 mm (wide gravel): 29 to 71 mm.
- 30 mm (mountain): 47 to 71 mm.
On hookless rims, Schwalbe points out that you can only mount tubeless tires (TLE or TLR), even if you run a tube, and that the wheel maker’s compatibility list, minimum width and maximum pressure take precedence. For pressure, use the bike tire pressure calculator.
Also, a tire grows when mounted on a wide rim: by our estimate, a 28 mm tire measures about 31 mm on a 25 mm rim. Make sure it fits your frame with room to spare.
Common mistakes
- Trusting the first inch number. “26 inch” can mean 559, 571, 584, 590 or 597 mm depending on the full size. Look for the ETRTO.
- Mixing up 650b and 650c. There’s a 13 mm difference in rim diameter: they’re not interchangeable.
- Assuming 27-inch and 700c are the same. A 27 × 1 1/4 tire needs a 630 mm rim; 700c is 622 mm. The 8 mm gap means one won’t mount on the other’s rim.
- Not updating the circumference after changing tires. If your bike computer is set for a 700×23C (2,096 mm) and you fit a 700×32C (2,155 mm), it will read almost 3% low on distance and speed.
- Using the nominal width to check frame clearance. Measure the tire once mounted and inflated.
Changed tires? The outer diameter changes your gearing: check it with the gear ratio calculator.
Frequently asked questions
What does 700x28c mean on a bike tire?
It's the French sizing: 700 is the nominal diameter, 28 the width in mm and the C indicates the 622 mm rim. In ETRTO it's a 28-622. The letters A, B and C were three different rim diameters for the same 700 or 650 size.
How do I convert tire width from inches to mm?
Multiplying by 25.4 gets you close, but manufacturers use their own equivalents: a 2.4 in tire is labeled 62 mm (not 61) and a 1.75 in, 47 mm (not 44). The converter applies the Schwalbe and ETRTO convention, so it gives you the number you'll see on the sidewall.
Is 650b the same as 27.5?
Yes: both use a 584 mm rim. Mountain bikes call it 27.5 in; gravel and classic bikes call it 650b. Don't confuse it with 650c, which is 571 mm and was used on small triathlon bikes.
What wheel circumference should I enter in my bike computer?
The one for your size in the chart on this page (700×25C: 2,105 mm; 700×28C: 2,136 mm; 29×2.2: 2,298 mm). To get it exact, mark the tire, roll one full revolution while sitting on the bike and measure the distance.
What tire width fits my rim?
Under the 2024 ETRTO standard, a rim with a 21 mm inner width takes 25 to 65 mm tires, a 25 mm rim 29 to 71 mm and a 30 mm rim 47 to 71 mm. On hookless rims, the wheel maker's list also applies.