How to Calculate a Motor Torque

Written by jessica reed
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How to Calculate a Motor Torque
("Car." [Online image] Available, 17 June 2009.)

Torque is a way of measuring how powerful a car's motor is. You may have heard of horsepower, but torque is important too. There are two common methods of calculating the torque of a motor. You can calculate torque or full-load torque. Full-load torque takes horsepower into account and assumes the engine is working at maximum power. Learn the formulas you'll need to calculate torque and full-load torque, and how to apply them to a motor.

Skill level:


    Calculating Torque

  1. 1

    Use the formula "T = F x D" where "T = torque" in ft-lb, which means the weight in pounds (lb.) being moved over the distance of one foot (ft.), "F = force" in pounds (lb.), and "D = distance" in feet (ft.)

  2. 2

    Replace the letter "F" with the force being applied. Replace the letter "D" with the distance that must travel for one rotation. For example, 27.2 Kilogram of force pushing on a lever arm that is 3 feet long would result in the formula "T = 60 x 3."

  3. 3

    Multiply "F" by "D" to get your answer. For this equation, the answer is "T = 180." The torque is 180ft-lb. This means this particular device can move 81.6 Kilogram per foot.

    Calculating Full-Load Torque

  1. 1

    Use the formula "T = HP x 5252 divided by RPM" where "HP" = horsepower and "RPM" = revolutions per minute.

  2. 2

    Replace "HP" with the horsepower of the motor and "RPM" with the rpm of the motor. For example, a 30 horsepower motor working at 1725 revolutions per minute gives you the formula "T = 30 x 5252 divided by 1725".

  3. 3

    Multiply the horsepower by 5252, in this case "30 x 5252" and divide your answer by RPM. For this equation, "30 x 5252" divided by "1725" equals 91.34. Your answer is "T = 91.34ft-lb". The full-load torque of the engine is 91.34 foot-pounds.

Tips and warnings

  • Foot-pounds may also be expressed as lb-ft instead of ft-lb.

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