The moment of inertia is expressed as Wk2or WR2in terms of pound-feet squared. It is the product of the weight of the object in pounds and the square of the radius of gyration in feet.

If the application is such that the motor is driving through a pulley or gear so that the driven equipment is operating at a higher or lower speed than the motor, it is necessary to calculate the inertia "reflected to the motor shaft," that is, an equivalent Wk2(reflected to motor shaft) = Wk2 based on the rpm of the motor.