Gear Module Calculator

How it is calculated

The module is the pitch diameter per tooth: m = PD / N, with PD the pitch diameter in millimetres and N the number of teeth. Two related values follow from it: circular pitch = π × m (mm, the distance along the pitch circle from one tooth to the next) and diametral pitch = 25.4 / m (teeth per inch of pitch diameter). Results are shown to 3 decimals. The pitch diameter must be above zero and the tooth count at least 1. The tool does not check that the result is a standard module, and the pitch diameter is not the same as the outside (tip) diameter, so use the right one.

Common mistakes

• Entering the outside (tip) diameter instead of the pitch diameter. For a standard gear the outside diameter is larger than the pitch diameter, so the module comes out too high.
• Entering the pitch diameter in inches. The module is in millimetres, so convert the diameter first.
• Mixing up module and diametral pitch. They are inversely related: a larger module means bigger teeth, while a larger diametral pitch means smaller teeth.
• Assuming two gears with the same tooth count will mesh. They must share the same module, so check the module before pairing gears.
• Rounding the module to a standard value too early. If the calculated value is far from a standard size, re-check the measurement instead of snapping to the nearest one.