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How to Calculate Gear Box Ratio

By Andrea Helaine
A gearbox is used after the motor to change its RPM speed.

The gear box ratio is the difference between the RPM produced by the motor and the RPM measured after the gearbox. A gear box can either increase or decrease the RPM of a motor. A gear box ratio is noted A:B where A>B in a speed reducer and A<B in a multiplier.

Write down the rated RPM of the motor. RPM_motor = 10,000RPM in our example.

Turn the motor on and use a tachometer to measure the RPM after the gearbox. Write that number down. RPM_gearbox = 2500RPM in our example.

Divide the RPM_motor by RPM_gearbox and write down that number. In our example, 10000/2500 = 4 so our ratio is 4:1.

Simplify the fraction RPM_motor/RPM_gearbox instead of calculating the ratio.


Things You Will Need

  • Motor RPM
  • Tachometer


  • The RPM value given by the motor manufacturer always come with a +/- value. That value tells you that the RPM speed varies by that percentage. For example, a 1000RPM motor with a +/- .5% value actually turns between 995 and 1005 RPM. Because of that fact, the RPM value after the gearbox may vary slightly and give you an odd number, for example 4.089:1. If it is the case, round that number to get a clean ratio of 4:1.


  • Do not touch the shaft of a running motor.

About the Author


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.