The gear ratio is expressed as a quotient, for example as 3:1 (read as "three to one"), indicating that the input gear must complete three full rotations to make the output gear turn one full rotation.

For example, if the driving gear has 20 teeth and the driven gear has 40 teeth, the gear ratio is: \ [ GR = frac {40} {20} = 2 \] This means the driven gear rotates once for every two rotations of the driving gear.

When a gear train has multiple stages, the gear ratio for the overall gearing system is the product of the individual stages. For example, for the gear at left the blue gears are 7 and 21 teeth, while the green gears are 9 and 30 teeth. Thus, the first gear ratio is 7:21 and the second is 9:30.