A gear ratio compares the rotation of two meshed gears. It’s calculated by dividing the number of teeth on the driven (output) gear by the number on the driving (input) gear.
Worked example
A 10-tooth driving gear turning a 40-tooth driven gear gives 40 ÷ 10 = 4, or a 4:1 ratio — the input turns four times for every one turn of the output.
What the ratio tells you
A ratio greater than 1 is a reduction: the output turns slower but with proportionally more torque — ideal for moving heavy loads or climbing hills. A ratio less than 1 is an overdrive: faster output, less torque. Speed and torque trade off inversely, so a 4:1 reduction quarters the speed and (ideally) quadruples the torque. For gear trains with multiple stages, multiply the individual ratios together to get the overall ratio.
Frequently asked questions
What is a 4:1 gear ratio? The input turns 4 times per 1 output turn — a reduction with 4× the torque.
Does a higher ratio mean more torque? Yes — more reduction trades speed for torque.
How do I find the ratio of a gear train? Multiply the ratios of each meshing stage.