HP & Torque Lab

James Watt coined "horsepower" in 1782 to market his steam engines against the draft horses they replaced. He measured a typical brewery horse lifting 33,000 foot-pounds of load per minute — rounding generously, since actual average horses manage closer to 22,000 ft-lb/min over an 8-hour shift. The 33,000 ft-lb/min figure became the imperial mechanical horsepower standard still used today (550 ft-lb/s).

The famous "5252 constant" in HP = (torque × RPM)/5252 falls out of the unit conversion: 33,000 ft-lb/min ÷ 2π radians/rev = 5252.1 ft-lb/rev. Its practical consequence: on any dyno graph, the HP and torque curves always cross at exactly 5252 RPM, because that's the RPM at which 1 lb-ft = 1 HP. This single constant is the reason HP overtakes torque past 5252 RPM for engines that keep breathing.

Metric horsepower (PS in Germany, CV in France/Spain, pk in the Netherlands) is slightly different: 75 kg-m/s = 735.5 W, versus imperial 745.7 W. A "300 PS" BMW and a "300 HP" Ford differ by about 1.4%. Modern automakers often quote both or state compliance with the SAE J1349 "certified net" measurement standard, which specifies atmospheric corrections, accessory loads, and fuel grade.

Enter any two of horsepower, torque (lb-ft), and RPM; the third computes automatically via HP = (torque × RPM) / 5252. Metric equivalents (Nm, kW, PS) are shown in parallel. At 5252 RPM, HP numerically equals torque — the crossover point that every dyno chart illustrates.

Remember this is engine crank power. Wheel HP from a chassis dyno is typically 10-20% lower due to drivetrain losses (manual transmissions lose ~15%, autos ~20-25%, AWD systems higher). For honest drag-strip predictions, use wheel HP and add a driver-reaction time estimate. Everything runs client-side; no values leave your browser.