The BikeRatio column comes from the same formula as our tire size converter: circumference = π × (rim diameter + 2 × 0.97 × width), because a mounted, inflated tire ends up about 97 % as tall as its nominal width. The CatEye column is its official chart for bike computers, published in 2015. Where CatEye doesn’t list a size, we show ”—”. Bike computers take circumference in millimeters, so this chart stays metric.
Road, gravel and touring (700C)
| Size | ETRTO | BikeRatio circumference | CatEye chart |
|---|---|---|---|
| 700 × 18C | 18-622 | 2,064 mm | 2,070 mm |
| 700 × 20C | 20-622 | 2,076 mm | 2,086 mm |
| 700 × 23C | 23-622 | 2,094 mm | 2,096 mm |
| 700 × 25C | 25-622 | 2,106 mm | 2,105 mm |
| 700 × 28C | 28-622 | 2,125 mm | 2,136 mm |
| 700 × 30C | 30-622 | 2,137 mm | 2,146 mm |
| 700 × 32C | 32-622 | 2,149 mm | 2,155 mm |
| 700 × 35C | 35-622 | 2,167 mm | 2,168 mm |
| 700 × 38C | 38-622 | 2,186 mm | 2,180 mm |
| 700 × 40C | 40-622 | 2,198 mm | 2,200 mm |
| 700 × 42C | 42-622 | 2,210 mm | 2,224 mm |
| 700 × 44C | 44-622 | 2,222 mm | 2,235 mm |
| 700 × 45C | 45-622 | 2,228 mm | 2,242 mm |
| 700 × 47C | 47-622 | 2,241 mm | 2,268 mm |
| 700 × 50C | 50-622 | 2,259 mm | — |
29” mountain bike
| Size | ETRTO | BikeRatio circumference | CatEye chart |
|---|---|---|---|
| 29 × 2.1 | 54-622 | 2,283 mm | 2,288 mm |
| 29 × 2.2 | 56-622 | 2,295 mm | 2,298 mm |
| 29 × 2.25 | 57-622 | 2,301 mm | — |
| 29 × 2.3 | 60-622 | 2,320 mm | 2,326 mm |
| 29 × 2.4 | 62-622 | 2,332 mm | — |
| 29 × 2.5 | 64-622 | 2,344 mm | — |
| 29 × 2.6 | 65-622 | 2,350 mm | — |
27.5” and 650B
| Size | ETRTO | BikeRatio circumference | CatEye chart |
|---|---|---|---|
| 26 × 1 1/2 (650B) | 37-584 | 2,060 mm | 2,100 mm |
| 650 × 38B | 40-584 | 2,078 mm | 2,105 mm |
| 650B × 47 | 47-584 | 2,121 mm | — |
| 27.5 × 1.95 | 50-584 | 2,139 mm | 2,090 mm |
| 27.5 × 2.1 | 54-584 | 2,164 mm | 2,148 mm |
| 27.5 × 2.25 | 57-584 | 2,182 mm | 2,182 mm |
| 27.5 × 2.4 | 62-584 | 2,213 mm | — |
| 27.5 × 2.6 | 65-584 | 2,231 mm | — |
| 27.5 × 2.8 | 70-584 | 2,261 mm | — |
26” and other adult sizes
| Size | ETRTO | BikeRatio circumference | CatEye chart |
|---|---|---|---|
| 26 × 1.50 | 40-559 | 2,000 mm | 2,010 mm |
| 26 × 1.75 | 47-559 | 2,043 mm | 2,023 mm |
| 26 × 1.95 | 50-559 | 2,061 mm | 2,050 mm |
| 26 × 2.10 | 54-559 | 2,085 mm | 2,068 mm |
| 26 × 2.125 | 57-559 | 2,104 mm | 2,070 mm |
| 26 × 2.35 | 60-559 | 2,122 mm | 2,083 mm |
| 26 × 3.00 | 75-559 | 2,213 mm | 2,170 mm |
| 650 × 23C | 23-571 | 1,934 mm | 1,944 mm |
| 26 × 1 3/8 | 37-590 | 2,079 mm | 2,068 mm |
| 27 × 1 1/4 | 32-630 | 2,174 mm | 2,161 mm |
Kids’ and folding bikes
| Size | ETRTO | BikeRatio circumference | CatEye chart |
|---|---|---|---|
| 12 1/2 × 2 1/4 | 57-203 | 985 mm | — |
| 14 × 2.125 | 54-254 | 1,127 mm | — |
| 16 × 1.75 | 47-305 | 1,245 mm | 1,195 mm |
| 16 × 1 3/8 (Brompton) | 35-349 | 1,310 mm | — |
| 20 × 1.50 | 40-406 | 1,519 mm | 1,490 mm |
| 20 × 1.75 | 47-406 | 1,562 mm | 1,515 mm |
| 20 × 2.0 | 50-406 | 1,580 mm | — |
| 24 × 1.75 | 47-507 | 1,879 mm | 1,890 mm |
| 24 × 2.0 | 50-507 | 1,898 mm | — |
Which value to use
For 26 of the 36 sizes CatEye publishes, our formula lands within 1 % of its value, and for every 700C size, within 1.3 %. The bigger gaps show up on small wheels and on wide 26” and 27.5” tires, where the tire’s real height strays further from what the formula assumes: on a 16 × 1.75 the gap is 4 %. If your size is in the CatEye chart, use that value; if not, use ours.
To get it truly right, do a rollout test at the pressure you usually ride: it’s the only method that accounts for your tire brand, your rim and your weight. Every 10 mm of error in circumference means half a mile of error every 100 miles, or about 0.1 mph at 25 mph.
- GPS bike computers. If your unit has GPS, the circumference is only used when it reads a speed sensor, for example on an indoor trainer or with no satellite signal. It’s still worth setting correctly.
- Sizes that aren’t listed. First find your ETRTO size, the four- or five-digit number on the sidewall (for example, 28-622), in the tire size chart, then enter width and diameter in the converter, which calculates any combination.
Circumference also changes your gearing: you’ll find it in the gear ratio chart, calculated with these same circumferences, and explained in the bike gearing guide.
Frequently asked questions
What wheel circumference should I use for 700x28C?
Our formula gives 2,125 mm and the CatEye chart, 2,136 mm. Either works: the difference is 0.5 %, half a mile every 100 miles.
What about a 29 x 2.4?
About 2,332 mm. CatEye doesn't list it; its closest size, the 29 × 2.3 (60-622), gives 2,326 mm. With tires this wide it's worth doing a rollout test.
How do I measure my exact wheel circumference?
With a rollout test: put the valve at the bottom and mark the floor, get on the bike, roll straight ahead one full turn until the valve is back at the bottom, and measure the distance between the two marks in millimeters.