How to use it
- Type the teeth of the Front sprocket (chainring) teeth and the Rear sprocket (cog) teeth.
- Enter the Wheel diameter with the tyre fitted, in inches, and your Cadence in turns per minute.
- Read Gear inches, then Distance per crank turn and Speed at this cadence. The steps show each figure.
How are gear inches calculated?
Gear inches say how far one turn of the pedals takes the bike, in terms of a wheel of the given size. In Sheldon Brown's definition, the diameter of the drive wheel is multiplied by the front sprocket and divided by the rear sprocket. A higher figure is a harder gear: each turn covers more ground, but it takes more force to turn the pedals.
Development is the same idea in metres. It is the wheel's circumference, pi times the diameter, multiplied by the ratio of front to rear teeth. The speed then follows from the cadence.
Gear inches = wheel diameter (in) × front teeth ÷ rear teeth
Distance per crank turn (m) = π × wheel diameter (in) × 0.0254 × front ÷ rear
Speed (km/h) = distance per crank turn × cadence × 60 ÷ 1000What does the calculator leave out?
- Tyre pressure and width change the real rolling diameter. The result uses the diameter you enter.
- Chain wear and drivetrain losses are not counted.
- Gear inches compare gears on one wheel size. A different wheel needs its own diameter entered.
Example: 50 front teeth, 17 rear teeth, 27-inch wheel
Gear inches: 27 × 50 ÷ 17 = 79.4. The distance per crank turn is 6.34 m, and at 90 turns a minute the speed is 34.2 km/h.
| Figure | Result |
|---|---|
| Gear inches | 79.4 |
| Distance per crank turn | 6.34 m |
| Speed at 90 rpm | 34.2 km/h |
Frequently asked questions
What is a good gear inch figure?
There is no single right figure. Lower numbers make climbing easier, while higher numbers cover more ground per turn on flat roads or downhill.
Why is the wheel diameter entered in inches?
Gear inches are defined in inches. The metre figure converts the diameter you enter at 0.0254 metres an inch.
Does the tyre width change the result?
Yes. A wider or more inflated tyre has a larger rolling diameter, so the real distance per turn and speed are a little higher than the figure you enter.
Can I use it for a fixed-gear or single-speed bike?
Yes. Enter the same number of teeth on the front and the rear sprocket, and the gear inches equal the wheel diameter.
Is the data I enter stored?
No. The calculation runs in your browser, and the values you enter are not sent or saved anywhere.
Sources
Figures and text last checked on Tuesday, November 10, 2026.