How do I find my bike wheel circumference?
Read the ETRTO size on the tire's sidewall, something like 28-622. The second number is the rim's diameter in millimeters, and the first is the tire's width, which it adds twice: once at the top of the wheel and once at the bottom. Multiply by π: (622 + 2 × 28) × 3.1416 = 2,130 mm. That's the number a bike computer's "wheel size" or "circumference" setting wants. The calculator above reads any notation (700 × 28C, 29 × 2.4, 26 × 1⅜) and corrects for rims wider or narrower than the tire's label assumes. The chart above covers the common sizes.
Why do wheel circumference charts disagree?
Because the familiar charts in computer manuals aren't formulas. They're old lists of measured values, passed from one manual to the next. Here is Garmin's published chart beside plain geometry:
| Tire | Garmin chart | (BSD + 2 × width) × π | Difference |
|---|---|---|---|
| 700 × 23C | 2,096 mm | 2,099 mm | +0.1% |
| 700 × 25C | 2,105 mm | 2,111 mm | +0.3% |
| 700 × 28C | 2,136 mm | 2,130 mm | -0.3% |
| 700 × 30C | 2,146 mm | 2,143 mm | -0.2% |
| 700 × 32C | 2,155 mm | 2,155 mm | 0.0% |
| 700 × 35C | 2,168 mm | 2,174 mm | +0.3% |
| 700 × 38C | 2,180 mm | 2,193 mm | +0.6% |
| 700 × 40C | 2,200 mm | 2,205 mm | +0.2% |
| 700 × 45C | 2,242 mm | 2,237 mm | -0.2% |
| 29 × 2.1 | 2,288 mm | 2,289 mm | +0.1% |
| 29 × 2.3 | 2,326 mm | 2,321 mm | -0.2% |
| 26 × 1.95 | 2,050 mm | 2,067 mm | +0.8% |
| 26 × 2.1 | 2,068 mm | 2,091 mm | +1.1% |
Garmin Edge owner's manual, "Wheel Size and Circumference". Geometry at the labeled width. This agreement is pinned in our test suite.
Every 700c and 29-inch row agrees with geometry to within 0.6%, and the 26-inch rows run up to 1.1% shorter than geometry. Either is close enough for most riders. Your actual wheel also depends on the rim, the brand of tire and how worn it is, which no chart can know. The chart gets you within about 1%; a roll-out gets you exact.
Does rim width change the circumference?
Yes. A tire on a wider rim stands wider and taller than its label, by about 0.4 mm of width for every extra millimeter of rim, the same width model our tire pressure calculator uses. For the same 700 × 28:
| Rim internal width | Tire stands at | Circumference |
|---|---|---|
| 15 mm | 26.4 mm | 2,120 mm |
| 17 mm | 27.2 mm | 2,125 mm |
| 19 mm | 28.0 mm | 2,130 mm |
| 21 mm | 28.8 mm | 2,135 mm |
| 23 mm | 29.6 mm | 2,140 mm |
| 25 mm | 30.4 mm | 2,145 mm |
Road tires are labeled for a 19 mm internal rim. Height taken as equal to the width the tire stands at.
That's 25 mm (1.2%) between an old narrow rim and a modern wide one, bigger than the error in most charts. If you've moved to wide carbon wheels, the number you set years ago now reads slightly short.
How much does the wrong wheel size setting matter?
Every percent of error is a percent off every speed and every mile the sensor records. The worst common case is a computer still set to 2,096 mm, the 700 × 23 value at the top of every old chart, on a bike with bigger tires:
| Actual tire | Real circumference | Error at 2,096 | 100 real miles show as | 20 mph shows as |
|---|---|---|---|---|
| 700 × 25C | 2,111 mm | -0.7% | 99.3 | 19.9 mph |
| 700 × 28C | 2,130 mm | -1.6% | 98.4 | 19.7 mph |
| 700 × 32C | 2,155 mm | -2.7% | 97.3 | 19.5 mph |
| 700 × 40C | 2,205 mm | -5.0% | 95.0 | 19.0 mph |
| 650 × 47B | 2,130 mm | -1.6% | 98.4 | 19.7 mph |
| 29 × 2.25 | 2,313 mm | -9.4% | 90.6 | 18.1 mph |
| 29 × 2.4 | 2,337 mm | -10.3% | 89.7 | 17.9 mph |
Real circumference at the label's standard rim. A computer that counts wheel turns multiplies each by the circumference you set, so the error is the ratio of the two, applied to everything.
Pair a speed sensor to a mountain bike and leave the default, and a 29 × 2.25 reads 9.4% low: a 30-mile ride logs as 27.2, and cruising at 20 mph shows 18.1. It also drags down every average speed, and anything computed from speed, such as estimated power, calories and ride goals, inherits the error.
Pump the tires to your riding pressure. Roll the bike until the valve sits at the very bottom and mark the floor beside it. Sit on the bike in your riding position (a wall or a friend helps), and roll straight forward until the valve returns to the bottom — then keep going for four more turns. Mark the floor again, measure the distance in millimeters, and divide by five. Five turns shrink a 5 mm marking error to 1 mm. Use the rear wheel if your sensor is on the rear hub.
Do tire pressure and my weight change it?
Less than you'd think. The tire flattens where it meets the road, but its tread is a nearly inextensible band, and a wheel moves forward one tread-length per turn whether the bottom is round or flattened. Softer tires and heavier riders shorten the roll-out slightly, and that's exactly why it's done sitting on the bike at riding pressure: the measurement captures the load, the pressure, the rim and the particular tire all at once, and the calculator's roll-out mode turns it straight into the setting.
Do I need this if my computer uses GPS?
For distance outdoors, mostly not: GPS head units measure distance from satellite position, and many use GPS to auto-calibrate the wheel size of a paired speed sensor. The wheel sensor still matters in three places: under tree cover and in city canyons where GPS wanders, on a smart trainer or rollers where the bike doesn't move, and on older or simple computers that only count wheel turns. If your unit offers "auto" wheel size, it's a good default, but check it once against a roll-out: auto-calibration learns from GPS, noise included. For what circumference does to your gearing, the gear ratio calculator turns it into speed at every cadence.