How to use the Horsepower
- To calculate from an engine curve, enter torque and the RPM at which it is measured.
- To estimate from performance, enter vehicle weight and trap speed.
- Note whether the figure is at the crank or the wheels.
- Compare across engines only when the measurement point matches.
How the calculation works
Power is torque times rotational speed. In imperial units, HP = torque (lb-ft) × RPM ÷ 5,252, where the constant is 33,000 ÷ 2π; in metric, kW = torque (Nm) × RPM ÷ 9,549. The two curves cross at 5,252 RPM by definition, which is why power keeps rising after torque peaks — the RPM multiplier grows faster than torque falls, until it doesn't.
Trap-speed estimation inverts the physics: power needed scales with weight and roughly the cube of speed, giving HP ≈ weight × (trap speed ÷ 234)³. It measures power at the wheels as delivered, so it lands 12 to 15% below a crank figure because of drivetrain losses. Crank, wheel and different regional rating standards all produce different numbers for the same engine, which is why quoted figures need their measurement point stated to be comparable.
HP = torque(lb-ft) × RPM / 5252 ; kW = Nm × RPM / 9549 ; HP ≈ weight × (trap mph / 234)³Source: Mechanical power definition (1 hp = 33,000 ft·lb/min); Huntington/Hale trap-speed approximation with the 234 constant.
Worked example
An engine making 310 lb-ft at 4,400 RPM, and a 3,650 lb car trapping 108 mph in the quarter mile.
- HP = 310 × 4,400 / 5,252 = 259.7 hp at that point.
- Trap estimate: (108 / 234)³ = 0.0983.
- HP ≈ 3,650 × 0.0983 = 359 hp at the wheels.
About 260 hp at 4,400 RPM from the torque figure, while the trap speed implies roughly 359 hp at the wheels near peak — around 415 hp at the crank allowing for drivetrain loss.
Frequently asked questions
What is the difference between horsepower and torque?+
Torque is twisting force; horsepower is torque combined with how fast it is delivered. Torque determines how hard the car pulls at a given RPM, power determines how quickly it can keep doing so.
Why do the power and torque curves cross at 5,252 RPM?+
Because the conversion constant is 5,252. At that speed the two numbers are numerically equal by definition, regardless of the engine.
Why is wheel horsepower lower than crank horsepower?+
Transmission, differential and tyre losses consume roughly 12 to 15% for a manual and more for an automatic. Compare figures only when both are measured at the same point.
How accurate is the trap-speed estimate?+
Within about 5% for a reasonably standard car on a good surface. It assumes typical aerodynamics and a clean run, so poor traction or unusual drag will skew it.
Last reviewed August 31, 2026. We review this page whenever the underlying formula, tax year, published rate or standard changes.