The horsepower formula
One mechanical horsepower equals 745.7 watts, so hp = watts ÷ 745.7. A 746 W motor is about 1.00 hp. Horsepower is a unit of power, the rate of doing work. To see the work itself, use the work calculator.
The number is historical rather than arbitrary: it came from an estimate of how much work a draught horse could do steadily, and it was later fixed as an exact multiple of the watt so the conversion is a simple division.
How to use this calculator
Enter the power in watts. The calculator divides by 745.7 to give mechanical horsepower (hp). This uses the standard imperial/mechanical horsepower; metric horsepower (PS) is slightly different at about 735.5 W. For machine efficiency, see the efficiency calculator.
If your figure is in kilowatts, multiply by 1,000 first — a 1.5 kW motor is 1,500 W. The commonest mistake is comparing an hp rating against a PS rating without noticing the difference, which quietly overstates a European engine by about 1.4%.
Worked example
A motor rated at 746 W: 746 ÷ 745.7 = 1.00 hp. A 1500 W motor would be about 2.01 hp, since horsepower scales linearly with power in watts.
Interpreting a horsepower figure means remembering it is a rate, not a total. A 1 hp motor and a 2 hp motor can do the same job, but the larger one does it in half the time. It also helps to know whether a quoted figure is input power drawn from the supply or useful output power at the shaft; the two differ by the machine's efficiency, and marketing figures often quote the flattering one. At the crank, power is torque times rotational speed, so the torque calculator explains where the number comes from.
Frequently asked questions
- How do I convert watts to horsepower?
- Divide the power in watts by 745.7: hp = watts ÷ 745.7.
- How many watts are in one horsepower?
- About 745.7 watts for mechanical horsepower.
- Is metric horsepower the same?
- No. Metric horsepower (PS) is about 735.5 W, slightly less than mechanical horsepower.
- What is horsepower a measure of?
- Power, which is the rate at which work is done or energy is transferred.
- How do I convert horsepower back into watts?
- Multiply the horsepower figure by 745.7. Two horsepower is 2 × 745.7 = about 1,491 W, or roughly 1.5 kW. That reverse step is handy when comparing an imperial-rated tool with an appliance labelled in kilowatts.