Bicycle Gear Ratio Calculator
Compare chainring and cassette combinations by their simple mechanical ratio.
How to use this calculator
- Enter the tooth count of the active front chainring and rear cassette or freewheel cog.
- Use the mechanical ratio to compare combinations on the same bicycle. A larger chainring or smaller rear cog raises the ratio and increases distance traveled per crank revolution.
- For comparisons across different wheel sizes, continue to the gear-inches calculator because simple tooth ratio does not include wheel diameter.
- When comparing climbing gears, focus on the lowest available ratio; when comparing high-speed gearing, focus on the highest ratio and your sustainable cadence.
- Remember that gearing changes leverage but not rider power. Gradient, aerodynamic drag, tire resistance and rider fitness determine how usable a theoretical gear actually feels.
Formula and assumptions
Bicycle mechanical gear ratio = chainring teeth ÷ rear-cog teeth. On a conventional chain drive, this is also the approximate number of rear-wheel revolutions per crank revolution before accounting for hub gearing or other internal stages.
GIIRR calculates idealized geometry and kinematics. Friction, slip, compliance, tire deformation, manufacturing tolerances and control-system behavior can move real measurements away from the theoretical result.
Worked example
A 50-tooth chainring with a 17-tooth rear cog gives 50 ÷ 17 = 2.94. Switching only the rear cog to 25 teeth reduces the ratio to 2.00, so the wheel turns about 32% fewer revolutions per crank turn. That makes the gear easier but also reduces road speed at the same cadence and wheel size.
Frequently asked questions
Is a lower ratio easier uphill?
Yes. A lower mechanical ratio gives more leverage at the rear wheel for the same crank torque, at the cost of less road distance per pedal revolution.
Why use gear inches too?
Gear inches include wheel diameter, so they allow more meaningful gearing comparisons between bicycles with different wheel and tire sizes.
Does crank length affect the gear ratio?
Not the tooth-count ratio. Crank length changes the rider's leverage and biomechanics, but the chainring-to-cog rotational ratio stays the same.
Can I use this for an internal-gear hub?
Only for the external chainring/cog stage. Multiply by the selected hub's internal ratio if you want the complete drivetrain ratio.
Why do two bikes with the same 50/17 ratio feel different?
Wheel diameter, crank length, bicycle mass, tire resistance, rider position, terrain and drivetrain losses can all differ even though the tooth ratio is identical.
For safety-critical, legal or fitment decisions, verify the result against manufacturer documentation and physical measurements.
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