Gear Ratio Calculator
Enter the tooth counts of two meshing gears and an input speed to get the gear ratio and the resulting output shaft speed — and see whether the pair is a reduction or an overdrive.
Count both gears
The ratio depends only on the tooth counts of the two meshing gears — count the teeth on the driver (input) and the driven (output) gear carefully.
What is a gear ratio?
The trade between speed and torque
A gear ratio is the relationship between how fast two meshing gears turn, set entirely by their tooth counts. This gear ratio calculator divides the driven (output) tooth count by the driver (input) tooth count to give the ratio, then uses your input speed to work out how fast the output shaft turns. It is the number behind every bicycle gear, drill gearbox, and car transmission: it tells you whether the gear pair trades speed for torque or torque for speed, and by how much.
Enter the driver teeth, the driven teeth, and an input speed in rpm to get the gear ratio and the output shaft speed instantly.
The gear ratio is the driven tooth count divided by the driver tooth count, and the output speed is the input speed scaled by the inverse of that ratio.
ratio = driven teeth ÷ driver teethWith a 12-tooth driver and a 36-tooth driven gear, the ratio is 36 ÷ 12 = 3:1. At an input speed of 1000 rpm the output shaft turns at 1000 × 12 ÷ 36 ≈ 333.3 rpm — a third of the input speed, with roughly three times the torque. A ratio above 1 is a reduction (more torque, less speed); a ratio below 1 is an overdrive (more speed, less torque).
The formula is exact for an ideal gear pair, but a few practical points are worth keeping in mind.
Ideal two-gear mesh, no losses
This calculator covers a single pair of meshing gears and assumes an ideal, loss-free mesh. It does not model friction, backlash, or efficiency, so the real output torque is always a little lower than the ratio implies. For a gear train with several stages, multiply the ratios of each stage together. Idler gears between the driver and driven gear change the direction of rotation but not the overall ratio.