Calculate horsepower from torque and RPM, or convert between HP, kW, PS and ft·lbf/s — essential for automotive and mechanical engineering.
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Horsepower (HP) is a unit of power defined by James Watt in the 1780s as an equivalent to the work done by a horse. One mechanical horsepower = 550 foot-pounds per second = 745.7 watts. Watt defined it to market steam engines by comparing them to the horse power they replaced. The metric horsepower (PS) = 735.5 watts — slightly less than mechanical HP.
HP = (Torque in lb·ft × RPM) / 5,252. This constant (5,252) comes from unit conversions: HP = (T × 2π × RPM/60) / 550 = T × RPM / 5,252. In metric: kW = (Torque in N·m × RPM) / 9,549. Torque peaks at lower RPM (useful for pulling/towing); horsepower peaks at higher RPM (useful for top speed).
HP (mechanical horsepower, US): 745.7W. PS (Pferdestärke, metric horsepower): 735.5W. kW (kilowatts): 1,000W. bhp (brake horsepower): measured at the flywheel on a dynamometer — the actual usable output after engine friction losses (slightly less than indicated HP). 1 PS ≈ 0.9863 HP ≈ 0.7355 kW.
Torque is a rotational force — how hard the engine twists (measured in N·m or lb·ft). Power is the rate of doing work — how fast that force is applied (measured in kW or HP). High torque at low RPM makes a vehicle feel strong from a standstill (trucks, diesel engines). High power means the torque is sustained to high RPM (sports cars).
A dynamometer (dyno) measures actual engine output by applying a resistance to the drivetrain and measuring the force and speed simultaneously. A "rolling road" dyno measures at the wheels (wheel horsepower, whp), which is 10–20% less than flywheel bhp due to drivetrain losses from the gearbox, driveshaft and differential.
Interestingly, a horse cannot actually sustain one "horsepower" continuously. A horse sustains about 0.7 HP during a working day but can produce up to 15 HP in short bursts. Watt set 1 HP higher than a horse's typical sustained output to make his steam engines seem more attractive compared to horses, giving a comfortable safety margin.
As of 2024, the Bugatti Tourbillon produces approximately 1,800 HP from a naturally aspirated V16 plus three electric motors. The Rimac Nevera electric hypercar produces 1,914 HP. The SSC Tuatara claims 1,750 HP. In Formula 1, current hybrid power units produce approximately 1,000 HP total from the 1.6L V6 turbo plus energy recovery systems.
Manufacturers measure power at the flywheel (crank HP), while wheel dynos measure after drivetrain losses. A manual gearbox loses 10–15% power; automatic/DSG lose 10–12%. Other losses come from the differential and driveshafts. Additionally, manufacturers sometimes measure in ideal conditions (temperature, altitude, fuel quality) that differ from real-world conditions.
