For different units of power, torque, or angular speed, a conversion factor must be inserted into the equation. For example, if the angular speed is measured in revolutions instead of radians, a conversion factor of 2π must be added because there are 2π radians in a revolution:

 

power = torque x 2π rotational speed,

 

where rotational speed is in revolutions per unit time.

 

Some people use horsepower (imperial mechanical) for power, foot-pounds (lbf·ft) for torque and rpm's (revolutions per minute) for angular speed. This results in the formula changing to:

 

 

This conversion factor is approximate because the transcendental number π appears in it; a more precise value is 5252.113 122 032 55...