Cycling Gear Ratio to Speed Calculator
Turn your chainring, cog, cadence and wheel size into road speed, metres of development per pedal stroke and gear inches.
How it works
Ratio = chainring ÷ cog · Development = ratio × wheel circumference · Speed = development × cadence × 60 ÷ 1000
Each turn of the cranks turns the rear wheel by the gear ratio, and each wheel turn covers one circumference of ground — so metres per stroke is simply ratio × circumference. Multiplying by cadence gives distance per minute, and the unit conversion turns that into km/h. Gear inches, an older but still-used measure, is the ratio times the wheel diameter in inches, letting you compare gears across wheel sizes.
Worked example
A 50-tooth chainring on a 15-tooth cog is a 3.33 ratio. With a 2.105 m wheel circumference that is 7.02 m of development per pedal stroke. At 90 rpm: 7.02 × 90 × 60 ÷ 1000 ≈ 37.9 km/h.
Frequently asked questions
What wheel circumference should I use?
Measure your own by rolling one full tyre revolution, or use a chart: a 700×25c road tyre is about 2105 mm and a 700×28c about 2136 mm. Circumference changes with tyre width and pressure, so a measured value is most accurate.
What are gear inches?
Gear inches equal the gear ratio multiplied by the wheel diameter in inches — the effective diameter of a penny-farthing driven directly. Bigger numbers mean a harder, faster gear; typical road gears range from about 30 (easy climbing) to 120 (top-end sprint).
How does cadence change my speed here?
Speed is directly proportional to cadence in a fixed gear, so spinning from 80 to 100 rpm raises speed by 25%. Shifting to a larger chainring or smaller cog raises speed at the same cadence by increasing the ratio.
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This calculator is for educational purposes. Double-check important results before acting on them.