Gear Ratio Speed Calculator

Written by Great Medico

Updated September 25, 2026 •2 min read

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Gear ratios change rotational speed in a predictable way, but RPM is not the same as linear speed. Gear Ratio Speed Calculator connects tooth counts with ideal input/output RPM so you can see exactly what this page can—and cannot—calculate. As a quick check, doubling the output-gear tooth count while keeping input teeth and input RPM fixed should halve ideal output RPM; that verifies the inverse speed relationship before you rely on the displayed value.

What this calculator does

The Gear Ratio Speed Calculator links input/output tooth counts with input/output rotational speed. On this page, “speed” means RPM: there is no wheel, pulley, or pitch diameter field for converting rotation into linear speed.

How to use it

Enter input/output tooth counts and input RPM. The current fields support output rotational speed, not linear speed, so use the output-RPM metric accordingly.

How the calculation works

The implemented relation is r = Tout/Tin with RPMout = RPMin/r. These fields describe rotational speed; no wheel or pulley diameter is present on this page to convert RPM into linear road or belt speed.

Example

For 20 input teeth, 40 output teeth, and 1200 rpm input, the ratio is 2:1 and output speed is 600 rpm.

How to interpret the result

Interpret the supporting output RPM as rotational speed. With a 2:1 tooth ratio, ideal output RPM is half the input RPM; with a 0.5:1 ratio it is twice the input RPM. Do not convert that number to vehicle, belt, or surface speed without a radius or circumference.

Limitations and notes

Despite the “Gear Ratio Speed” name, the current strict path does not compute linear speed and reports gear ratio as the headline. Use the supporting output-RPM value for rotational speed. To obtain linear speed you would still need an effective radius or circumference plus any additional transmission stages.