# How to Calculate the RPM of a Motor and Speed Reducer

by Andrea HelaineUpdated March 16, 2018Motors 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.

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

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

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

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.

### Items you will need

Motor base RPM

Speed reducer ratio

#### Tips

The ratio of a reducer will always be in the form of A:B, where A>B.

#### Warnings

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.

References

Tips

- The ratio of a reducer will always be in the form of A:B, where A>B.

Things You'll Need

- Motor base RPM
- Speed reducer ratio

Warnings

- 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.

Writer Bio

Andrea Helaine has a Bachelor of Philosophy in theology and is currently finishing her thesis course for a Master of Fine Arts in creative writing. Helaine has been writing professionally for over 10 years and has been published in several anthologies and is currently breaking into the screenwriting market.