Horsepower is a measure of power, and voltage measures the amount of energy carried in a circuit. Current, measured in amps, represents how fast the energy moves through a circuit. For example, you might use horsepower and voltage to find the current in a motor. In order to calculate the current from the horsepower and voltage, you must also know the efficiency of the circuit.

Multiply the amount of horsepower by 746. For example, if you had 4 hp, you would get 2,984.

Divide the result from step one by the voltage. For example, if the voltage were 5 volts, you would get 596.8.

Divide the efficiency by 100 to convert it from a percentage to a decimal. For example, if the efficiency were 85 percent, you would get 0.85.

Divide the result from step 2 by the efficiency of the motor to find the current. For example, if the efficiency expressed as a decimal is 0.85 efficient, you would divide 596.8 by 0.85 to determine the current equals about 702.118 amps.