Gear Ratio Guide

For a simple external spur gear pair, the ratio is just the tooth-count ratio:

ratio = teethdriven / teethdriver

A 40-tooth gear driven by a 20-tooth gear turns at half the speed and twice the torque (minus friction losses) of the driver — a 2:1 reduction. The direction of rotation reverses with every external mesh stage.

Center distance between two same-module gears is:

center distance = m × (z1 + z2) / 2 = (sum of the two pitch radii).

Every gear page on this site lists exactly this in its "Meshing" section for its two same-module neighbors, so you can read the center distance directly rather than recompute it — for example, the module 1, 20-tooth page shows its center distance against the neighboring 18- and 22-tooth gears at that module.

Large ratios: stack, don't stretch

A single spur pair much beyond about 1:6 tends to need an awkwardly large driven gear and wears faster because so few driver teeth share the load at any instant. For bigger reductions, chain two stages: two 1:3 pairs in series give 1:9 overall, using far more reasonably sized gears than one 1:9 pair would. This is the standard approach for hobby gearmotors and robotics gear trains — see the Hobby Robot Gears page for sizing notes.

Picking teeth counts

Where the ratio allows some freedom, prefer tooth-count pairs that don't share a common factor (e.g. 20:21 rather than 20:40 reduced badly, though that specific example isn't a real reduction) — a non-integer or coprime ratio means each tooth on one gear doesn't repeatedly hit the exact same tooth on the other, which spreads wear more evenly. This matters most for gears expected to run for a long service life.