The word horsepower dates to the 1700s, when steam-engine salesmen needed a way to tell farmers their machine could do the work of several draft horses. One mechanical horsepower was defined as 550 foot-pounds per second — and in metric terms that works out to exactly 745.7 watts. Today the unit is everywhere in American life: car engines, lawn mowers, shop vacuums, and generators all advertise their hp rating, while the electrical world bills you in watts and kilowatts. When those two worlds collide — sizing a generator for your house, comparing a gas mower to an electric one, or figuring out what a 20-amp circuit can actually power — you need the 745.7 bridge.

Type your number in the Horsepower (hp) box and the Watts (W) readout multiplies it by 745.7 instantly, with a bonus Kilowatts (kW) value at one-thousandth of that. A 100 hp engine becomes 74,570 W, or 74.57 kW. Try a real machine from your garage: a 6.5 hp lawn mower is about 4.85 kW, which explains why electric mowers quote similar kilowatt numbers. Remember that this page uses mechanical horsepower, the standard for engines and motors in the US. (Boiler horsepower and metric horsepower use slightly different factors, but engine spec sheets in America mean mechanical.)

With the pre-filled defaults — Horsepower (hp) 100 — the Horsepower to Watt returns Watts (W) 74,570.00, Kilowatts (kW) 74.5700. Change any input and those results update instantly, so you can see exactly how each number you enter moves the answer.