Gear Ratio Calculator
A gear ratio isn't just a number for comparing setups — it directly tells you the output shaft's speed once you know the input RPM, which is what most people actually need it for.
Inputs
Result
3 : 1
Output speed 500 RPM
How the gear ratio calculator works
Gear ratio = driven gear teeth ÷ driver gear teeth.
Output speed = input RPM ÷ gear ratio — a ratio greater than 1 (more teeth driven than driving) reduces speed; less than 1 increases it.
Worked example: driver 20 teeth, driven 60 teeth, input 1,500 RPM
- Ratio = 60 ÷ 20 = 3 : 1.
- Output speed = 1,500 ÷ 3 = 500 RPM.
- This is a reduction: the output spins one-third as fast as the input, but with three times the torque (ignoring losses), since gear systems trade speed for torque.
Common mistakes to avoid
Swapping which gear is 'driver' and which is 'driven'
The driver is the gear receiving input power (e.g. from a motor); the driven gear is turned by it. Swapping the two inverts the ratio and gives the reciprocal of the correct answer.
Assuming gear ratio alone tells you torque directly without accounting for efficiency
In an ideal gearbox, torque multiplies by the same ratio speed divides by, but real gearboxes lose some power to friction — actual torque output is slightly less than the ideal calculation suggests.
Frequently asked questions
What does a ratio like 3:1 actually mean in practice?
For every 3 rotations of the driver gear, the driven gear completes 1 rotation — the driven gear turns slower but with proportionally more torque, ignoring mechanical losses.
How do I calculate a gear ratio for more than two gears in sequence?
Multiply the individual ratios of each gear pair in the train together to get the overall ratio from first input to final output.
Does gear ratio affect torque as well as speed?
Yes — in an ideal system, torque changes by the same ratio speed does, but in the opposite direction: reducing speed by a factor of 3 roughly triples torque.
Is a gear ratio below 1 a 'reduction' or an 'increase'?
A ratio below 1 (fewer teeth on the driven gear than the driver) increases output speed above input speed — this is an overdrive, the opposite of a reduction.
Learn more
How to Calculate Gear Ratio: Formulas, RPM, and Gearbox Math
Master gear ratio calculations, output RPM conversions, torque leverage, and multi-stage gearbox speed formulas with clear worked examples.
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