About the Bike Gear Calculator
This calculator turns a chainring-and-cog combination into the numbers cyclists actually use to compare gears: gear ratio, gear inches, development (metres rolled per crank turn), and the resulting road speed at a chosen cadence. Whether you are choosing a cassette, planning a single-speed build, or working out whether you have a low enough climbing gear, these four values describe exactly how far one turn of the pedals carries you and how hard that turn will feel.
The formulas
- Gear ratio = chainring teeth ÷ cog teeth. It is a pure number: a 50-tooth chainring with a 15-tooth cog gives 50 ÷ 15 = 3.33, meaning the wheel turns 3.33 times for every crank revolution.
- Gear inches = gear ratio × drive-wheel diameter (in inches). This is a historical measure from the penny-farthing era — it is the diameter a direct-drive front wheel would need to give the same gearing. We derive wheel diameter from the circumference you enter: diameter = circumference ÷ π, converted to inches.
- Development (metres of travel per crank revolution) = gear ratio × wheel circumference. It answers "how far do I go for one full turn of the pedals?"
- Speed = development × cadence × 60 ÷ 1000 (km/h), where cadence is in crank revolutions per minute. Multiply crank rpm by the ratio to get wheel rpm, by circumference to get distance per minute, then scale to per hour.
Why gear inches instead of just the ratio?
Gear ratio alone ignores wheel size. A 2:1 ratio on a 700c road wheel and a 2:1 ratio on a 20-inch BMX wheel move you very different distances per pedal stroke. Gear inches folds wheel diameter in, so it lets you compare gearing fairly across bikes with different wheels. On the road, useful gears typically run from about 30 gear inches (a very low climbing gear) up to 120+ gear inches (a big sprinting or descending gear); most everyday riding sits between 55 and 90.
Common reference points
- 50 × 15 on a 700×25c wheel (circumference ~2105 mm): ratio 3.33, about 88 gear inches, 7.02 m development, ~37.9 km/h (23.5 mph) at 90 rpm — a typical fast-cruising road gear.
- 34 × 32 compact climbing gear on the same wheel: ratio 1.06, about 28 gear inches, 2.24 m development — the low bail-out gear for steep climbs.
- 44 × 16 single-speed: ratio 2.75, about 72.5 gear inches, 5.79 m development — a popular all-round fixed/single-speed ratio.
Wheel circumference reference values (mm)
- 700×23c road: ~2096 mm; 700×25c: ~2105 mm; 700×28c: ~2136 mm
- 26×2.1 MTB: ~2055 mm; 27.5×2.25 (650b): ~2182 mm; 29×2.2: ~2298 mm
- For best accuracy, measure your own with a rollout test rather than relying on a chart — pressure, load, and tire model all shift the real value by 1–2%.