Bike Gear Ratio Calculator

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.

Enter chainring teeth, rear cog teeth, cadence in RPM, and wheel circumference in meters. The calculator returns gear ratio, rollout per revolution, and speed in km/h and mph.

Worked example with defaults: 50/25 with 2.1 m wheel → ratio 2.0:1, rollout 4.2 m/rev. At 90 RPM → 22.7 km/h (about 14.1 mph). Dropping to a 28T cog lowers speed at the same cadence — easier climbing.

Measure wheel circumference by marking the tire, rolling one full revolution loaded with your weight, and measuring distance. Tire width and pressure change rollout by a few millimeters.

Cadence and speed scale linearly at a fixed gear: +10 RPM ≈ +11% speed. Shifting to a larger chainring or smaller cog has the same effect as pedaling faster in the previous gear.

Gravel and mountain bikes use smaller wheels with different circumferences — a 29-inch mountain tire may measure near 2.29 m rollout while a 700c road wheel stays near 2.1 m. Always measure rather than assume road defaults on off-road setups.

Indoor trainers display speed from power models or wheel speed — compare trainer speed to this calculator only when you know the simulated grade and wheel setup match your road bike configuration.

Compact cranksets (50/34) with wide-range cassettes (11–34) let you spin lower gears on climbs — run the calculator twice comparing big-ring flats versus granny-gear climbs to see speed at your preferred cadence in each.

Limits: no account for drivetrain efficiency, tire slip, or aerodynamic drag. Real-world speed on flats depends on power and wind, not gearing alone.