Bicycle Gear Calculator
Calculate external gear ratio, gear inches, development, and theoretical speed from chainring and rear-cog tooth counts, cadence, and measured loaded wheel geometry.
For the former compact workflows, use the gear-ratio preset or cadence preset.
Measure the wheel rollout
Inflate the tyre to normal riding pressure. Mark the tyre and ground at the valve, load the bicycle as it is ridden, roll straight for one complete wheel revolution, and measure between the marks. Enter that rollout as circumference.
Use diameter mode only for a directly measured loaded outside/effective diameter. Do not enter a nominal wheel-family label.
Method
For this direct external drivetrain, the gear ratio is chainring teeth divided by rear-cog teeth. Development is measured circumference multiplied by that ratio. Effective diameter is circumference divided by π, and gear inches is that diameter in inches multiplied by the ratio. Theoretical speed is development multiplied by cadence and converted from metres per minute to km/h and mph.
Calculations keep full precision until the displayed result. Enter valid numbers within the shown ranges. The calculator does not show a result when a value is missing, outside its range, or would make the calculation undefined.
Limits
The result is theoretical no-slip rollout. Tire slip, dynamic deformation, drivetrain loss, wind, grade, acceleration, and rider power are not modeled. It does not prescribe gearing, cadence, training, safety, or performance.
Sources
- Schwalbe tire dimensions guidance explains why actual dimensions vary and recommends measuring circumference.
- Wahoo manual rollout guidance and the CATEYE computer manual describe one-revolution measurement.
- Rohloff’s handbook gives the development relationship.
- NIST SP 811 provides the unit and rotational-frequency basis.