How to calculate the rpm of a motor and speed reducer

Written by andrea helaine
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How to calculate the rpm of a motor and speed reducer
In a speed reducer, the motor turns the small cog that turns the large cog to reduce the speed. (Large and small Cog Wheels together. image by daseaford from

Motors run at a specific revolutions per minute (RPM) and a speed reducer is needed if you want to keep that motor, but have a lower RPM. Calculating the RPM resulting from a motor and speed reducer assembly requires only basic mathematical knowledge.

Skill level:

Things you need

  • Motor base RPM
  • Speed reducer ratio

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  1. 1

    Write down the rated RPM of your motor; for example, 5000 RPM.

  2. 2

    Write down the ratio of the speed reducer; for example, 12:5.

  3. 3

    Calculate the reduction by dividing 12 by 5, which equals 2.4.

  4. 4

    The RPM of the assembly is the RPM motor divided by the reduction. In our example, it would be 5000 RPM/2.4 = 2083 RPM.

Tips and warnings

  • The ratio of a reducer will always be in the form of A:B, where A>B.
  • The RPM found should always be lower than the base RPM of the motor. If it is not, either redo your calculation or make sure you did not get a speed multiplier instead of a reducer.

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