Bike Gear Ratio Calculator
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Gear ratio, speed, and rollout from chainring, cassette cog, cadence, and wheel circumference.
Drivetrain
Speed
22.7 km/h
Gear ratio 2:1 @ 90 RPM
- Rollout
- 4.2 m/rev
- Speed (mph)
- 14.1
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Bicycle gearing
Gear ratio is chainring teeth divided by rear cog teeth — a 50/25 setup is 2.0:1. Rollout (distance per pedal revolution) equals wheel circumference × gear ratio. Speed follows from rollout and cadence (RPM).
Standard 700×25 mm road tires have roughly 2.1 m circumference — measure your own wheel for accuracy. Lower gears (smaller ratio) climb easier; higher gears go faster on flats.
Example: 50/25 with 2.1 m wheel → 4.2 m rollout. At 90 RPM cadence → 22.7 km/h (about 14.1 mph).
Common questions
What wheel circumference should I use?
700×25 mm tires ≈ 2.1 m. 700×28 mm ≈ 2.15 m. Measure rollout with a tape: one revolution loaded with your weight.
How does cadence affect speed?
Speed scales linearly with cadence at a fixed gear. +10 RPM ≈ +11% speed in the same gear.