How do you calculate horse power ?

Horsepower is a unit of power, so can be calculated as either work done per unit time, or force times velocity.

For example, to determine the horsepower output of an automobile engine, the car is typically mounted on a dynamometer which applies a predetermined resistance to the turning of the car's drive wheels. So what is actually measured is the torque in ft-lbs that the engine puts out through the wheels; this torque is then be multiplied by the speed of the engine (RPM) and a conversion factor to determine the car's horsepower at the wheels. The actual formula is:

HP = Torque (lb-ft) x engine speed (RPM)/5252