The formula to calculate horsepower is torque in pound-foot units multiplied by rotational speed and divided by 5,252. The formula changes when torque is in pound-inches to multiplying torque by rotational speed and dividing by 63,025.
Horsepower measures the rate at which work is done. Scottish engineer James Watt invented the term when he compared steam engines to draft horses, and the term currently applies to piston engines, turbines and electric motors as well as other types of machinery. Mechanical horsepower is 33,000 foot-pounds per minute or 550 foot-pounds per second. Most countries use the International System of Units watt when measuring horsepower.