How this instrument works
Every pedal revolution turns the wheel a fixed distance, set by the gear ratio (chainring teeth divided by cog teeth) and the wheel's circumference. Multiply that distance-per-revolution by cadence — revolutions per minute — and the result is a rate of travel: how far the bike covers in a minute, which converts to a familiar speed in kilometres per hour with one more unit-conversion step.
This is literally how a wired, magnet-based cyclocomputer calculates speed once a rider programmes in wheel circumference: it counts wheel revolutions directly rather than cadence, but the same distance-per-revolution logic sits underneath. This instrument instead starts from cadence and gear ratio, useful when working out what speed a given gear and pedalling rate would produce before ever getting on the bike.
The constant 0.06 in the formula does the unit conversion: 60 minutes per hour divided by 1,000 metres per kilometre. Change the gear ratio and the same cadence produces a different speed — spin the same 90 rpm in a taller gear and the bike covers more ground per revolution, which is exactly why shifting up increases speed at a constant pedalling effort.
- Enter Cadence (rpm) — your pedalling rate; 90 is a commonly used training reference point.
- Enter Gear ratio (chainring / cog) — divide the chainring's tooth count by the current cog's tooth count, e.g. a 50-tooth chainring on a 20-tooth cog is a ratio of 2.5.
- Enter Wheel circumference (m) — 2.1 m is a close approximation for a common 700×25c road wheel; measure your own tyre for precision.
- Read Speed (km/h) — the road speed that cadence and gear combination produces on a flat with no wind resistance factored in.
- All three inputs must be greater than zero, or the instrument will prompt for a valid value.
Worked example — 90 rpm in a 2.5 gear ratio
Enter 90 into Cadence (rpm), 2.5 into Gear ratio (chainring / cog) and 2.1 into Wheel circumference (m) — a rider spinning a moderate cadence in a mid-range gear on a standard road wheel. Speed reads 28.35 km/h.
That figure comes from 90 × 2.5 × 2.1 × 0.06 = 28.35: each pedal revolution advances the wheel 2.5 × 2.1 = 5.25 m, and at 90 of those revolutions a minute the bike covers 472.5 m per minute, which is 28.35 km every 60 minutes.
Questions
What is 'gear ratio' here, and how do I find mine?
Gear ratio is the chainring's tooth count divided by the current cog's tooth count on the cassette — a 50-tooth chainring paired with a 20-tooth cog gives a ratio of 2.5. Both counts are usually stamped or printed on the components themselves, or listed in the bike's or groupset's specification sheet.
How do I find my wheel's actual circumference instead of guessing?
Wrap a tape measure around the inflated tyre at riding pressure, or roll the wheel exactly one revolution along a marked straight line and measure the distance covered. Common road tyre sizes fall close to 2.1-2.2 m; mountain bike and gravel tyres run larger, so measuring your own setup gives a more accurate speed than relying on the default.
How is this different from the bike-gear-calculator instrument on this site?
This page converts cadence and gearing into an actual speed in km/h; bike-gear-calculator instead reports 'gear inches', a wheel-independent number used to compare gearing across completely different bikes without reference to any particular speed. Both use the same underlying gear-ratio idea, but this page answers 'how fast', while gear inches answers 'how does this gear compare to another'.
Why does the same cadence give different speeds in different gears?
Because each gear determines how far the wheel turns per pedal revolution — a taller gear (bigger chainring, smaller cog) advances the bike further for the same 90 rpm than a shorter gear does. Speed depends on cadence and gearing together, not cadence alone, which is exactly what this instrument's three inputs capture.
Does this account for wind resistance, hills or drafting?
No — this is a pure drivetrain calculation: given this cadence and this gear, the wheel must be covering this much ground per minute, full stop. Wind, gradient and drafting all change how much effort is needed to hold a given cadence in a given gear, but they don't change the mechanical relationship between cadence, gearing and speed that this formula describes.
Can I use this to measure my cadence first, if I don't already know it?
Yes — this site's bike-cadence instrument converts a counted number of pedal revolutions over a timed interval into rpm, which is exactly the cadence figure this calculator needs as an input. Measure cadence there, then bring that number here alongside a gear ratio and wheel circumference to get a speed.