How to calculate torque and rpm

Written by andi small | 13/05/2017
How to calculate torque and rpm
One horsepower rating is the equivalent of 33,000 foot-pounds per minute. (shire horse working image by lprendy from

Torque and revolutions per minute, or RPM, are physical concepts in rotational mechanics. Strictly defined, torque is the applied force scaled by the linear distance of a level arm, a value that quantifies how much force gives rise to rotational motion, such as that imparted by a wrench. RPM quantifies the periodicity of the motion. The product of torque and RPM is power, or the rate at which an object in rotational motion does work. You can calculate each of these values when the power rating is known, as is common with motorised engines.

Determine the frequency (f) of rotation. If the rotational motion of interest is that of a car engine, the object in rotational motion is the crankshaft. Do this by timing one full cycle from start to finish. The inverse of the amount of time (t) it takes to complete one cycle is the frequency (f = 1/t). Units of frequency are hertz (Hz) or inverse seconds.

Convert the frequency to RPM. Divide the frequency by 60 to denote the number of cycles per minute instead of per second, or RPM = f/60.

Write down the power rating of the engine. This will most likely be given in units of horsepower, or HP. The power rating is a universal indicator of the engine's performance, so it should be easy to find in manufacturer literature, such as the owner's manual.

Multiply the power by 746 watt/horsepower (W/HP) to write the power rating in terms of more standard units if that is preferred. Remember that a watt is a joule/second, so it'll be necessary to divide the power by 60 to put the work rate in "per minute" terms as well. Omit this step if the you want to render the torque in units of foot-pounds.

Divide the power by the RPM. The full formula for the power (P) in terms of the torque (T) and RPM is P = 2(pi)(T)(RPM), so you will also need to multiply the result by a factor of 2(pi) to get the correct torque. Check your answer by ensuring that your final value is given in units of foot-pounds or Newton-meters.

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