Gear Ratio RPM Calculator

Written by Great Medico

Updated September 25, 2026 •2 min read

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A short mechanics equation can tell you a lot—provided the sign convention and idealizations are clear. Gear Ratio RPM Calculator is built around gear ratio and output RPM, so the useful result stays connected to the motion or force relationship that produces it. As a quick check, doubling the output-gear tooth count while keeping the input gear and input RPM fixed should double the ratio and halve ideal output RPM.

What this calculator does

The Gear Ratio RPM Calculator links input gear teeth, output gear teeth, input rotational speed, and output rotational speed. Tooth counts establish the gear ratio, and that ratio determines the ideal output RPM from the entered input RPM.

How to use it

Enter input/output tooth counts and input RPM. Use the supporting output-RPM value after the ratio is calculated.

How the calculation works

For an input gear with Tin teeth and output gear with Tout teeth, the page uses ratio r = Tout/Tin and output speed RPMout = RPMin/r. It can also back-calculate compatible gear-tooth or speed values.

Example

With 20 teeth driving 40 teeth, r = 2. An input speed of 1200 rpm therefore gives an output speed of 600 rpm.

How to interpret the result

A ratio above 1 means the output gear has more teeth than the input gear, so ideal output RPM is lower by that same ratio. A ratio below 1 raises ideal output RPM. Use the calculated output-RPM field for speed and the ratio as the mechanical relationship between the two gears.

Limitations and notes

The current strict page reports the gear ratio as the primary headline, not output RPM, even though output RPM is calculated and shown as a supporting value. The model assumes an ideal gear pair and ignores slip, losses, backlash, idlers, compound stages, and direction of rotation.