Circumference in millimeters for 23 common sizes, computed as pi times (rim bead seat diameter plus twice the tire width). Set this correctly and every speed sensor gets honest.
Monsoon Cycles, Colorado Springs · Updated August 16, 2026
| Size | Circumference |
|---|---|
| 700c road | |
| 700c x 23 | 2093 mm |
| 700c x 25 | 2105 mm |
| 700c x 28 | 2124 mm |
| 700c x 32 | 2149 mm |
| 700c x 35 | 2168 mm |
| 700c x 38 | 2187 mm |
| 700c x 42 | 2212 mm |
| 700c x 45 | 2231 mm |
| 700c x 50 | 2262 mm |
| 650b / 27.5 | |
| 650b x 38 | 2073 mm |
| 650b x 42 | 2098 mm |
| 650b x 47 | 2130 mm |
| 27.5 x 2.1 | 2168 mm |
| 27.5 x 2.25 | 2193 mm |
| 26 and 29 MTB | |
| 26 x 1.5 | 1995 mm |
| 26 x 1.95 | 2070 mm |
| 26 x 2.1 | 2089 mm |
| 29 x 2.1 | 2287 mm |
| 29 x 2.25 | 2312 mm |
| Other | |
| 650c x 23 | 1938 mm |
| 27 x 1-1/4 | 2182 mm |
| 24 x 1.75 | 1869 mm |
| 20 x 1.75 | 1552 mm |
| 20 x 2.125 BMX | 1615 mm |
These are pi x (BSD + 2 x tire width): 700c has a 622mm bead seat, 650b is 584, 26 inch is 559. Real mounted tires squish and balloon a few millimeters either way by casing and pressure, so for laboratory truth do a rollout: mark the floor, roll one full wheel revolution seated on the bike, measure. For everything else this chart is within a percent.
Every speed sensor counts wheel revolutions. Circumference converts revolutions to distance, so an error here scales every speed and distance number your device reports.
Pick your tire size in the Wheel selector; the chart above is exactly what it uses.