Brake Horsepower Calculator
Brake horsepower (BHP) is calculated from engine torque (in lb-ft) and engine speed (RPM) using the fundamental relationship HP = Torque * RPM / 5,252. This formula comes directly from the definition of mechanical power and the way rotational motion converts to linear work. James Watt defined one horsepower as 33,000 foot-pounds of work per minute; dividing by 2*pi converts rotational torque (lb-ft) at a given RPM into those same units. The SAE (Society of Automotive Engineers) standardizes how this measurement is performed for published vehicle ratings.
Brake horsepower formula
HP = Torque (lb-ft) * RPM / 5,252 kW = HP * 0.745699872
The constant 5,252 = 33,000 / (2 * pi) = 5252.11. One metric horsepower (PS) = 0.98632 HP. One kilowatt = 1.34102 HP. At 5,252 RPM, torque (lb-ft) and HP are always numerically equal.
Solving for torque from HP
- Rearranging: Torque (lb-ft) = HP * 5252 / RPM.
- A 500 HP engine at peak power (6000 RPM): torque = 500 * 5252 / 6000 = 437.7 lb-ft.
- A diesel with 500 lb-ft at 2000 RPM: HP = 500 * 2000 / 5252 = 190.4 HP.
- This illustrates why diesel engines feel strong at low RPM despite modest peak HP ratings.
Frequently asked questions
What is the formula for horsepower from torque?
HP = Torque (lb-ft) * RPM / 5252. The constant 5252 comes from 33,000 ft-lb/min (James Watt's definition of one horsepower) divided by 2*pi (to convert between linear and rotational work). At exactly 5252 RPM, torque and horsepower are always equal.
What is brake horsepower (BHP)?
Brake horsepower is the power measured at the engine's crankshaft output, named after the brake dynamometer originally used to measure it. It equals wheel horsepower plus drivetrain losses. Most manufacturer-rated HP figures are SAE net BHP measured at the flywheel.
Why do torque and HP curves cross at 5,252 RPM?
Because of the formula: HP = Torque * RPM / 5252. When RPM = 5252, HP numerically equals torque. Below 5252 RPM, torque is higher; above 5252 RPM, HP is higher. This mathematical relationship is why performance engines spin their peak HP above the torque peak.
What is the difference between SAE gross and SAE net HP?
SAE gross (pre-1972) measured power with no accessories running (no alternator, no air cleaner, no muffler), inflating ratings. SAE net (1972-present) measures with all accessories installed as on a production vehicle. SAE net ratings are 20-30% lower and more realistic.
How do I find my engine torque?
Torque is measured on a dynamometer. Vehicle manufacturers publish torque curves in owner's manuals and press releases. Aftermarket dynos (chassis or engine dyno) can measure actual torque output after modifications. Torque gauges in modern vehicles estimate (not measure) from ECU data.
Official sources
- SAE International J1349: sae.org - Engine Power Test Code - Spark Ignition and Compression Ignition.
- NIST: nist.gov - unit definitions for power and torque.
Reviewed by the CalculatorHub team, edited by James Graham, 15 June 2026. See our methodology.