How to calculate horsepower
Horsepower is the rate of doing work: torque multiplied by how fast the engine turns. With torque in pound-feet and speed in RPM, the conversion constant is 5,252, which is also why torque and horsepower curves always cross at 5,252 rpm on a dyno chart.
Horsepower formulas
From elapsed time: HP = weight (lb) ÷ (ET ÷ 5.825)³
From trap speed: HP = weight (lb) × (trap mph ÷ 234)³
1 hp = 0.7457 kW = 1.0139 PS
Worked examples
- Dyno numbers: 350 lb-ft at 5,500 rpm = 350 × 5,500 ÷ 5,252 = 366.5 hp (273.3 kW).
- Drag strip time: a 3,500 lb car (with driver) running a 12.5 second quarter mile makes about 354 hp at the wheels.
- Trap speed: the same car trapping at 112 mph suggests about 384 hp at the wheels.
Crank vs wheel horsepower
Manufacturers quote horsepower at the crankshaft. Some power is lost through the transmission, driveshaft and differential before it reaches the wheels, commonly estimated at 10% to 20% depending on the drivetrain. Drag strip formulas and chassis dynos measure power at the wheels, so expect lower numbers than the brochure.
Want to predict your quarter mile from horsepower instead? Use the quarter mile calculator. Working on the engine? Try the engine displacement and compression ratio calculators.
Frequently asked questions
How do you convert torque to horsepower?
Multiply torque in pound-feet by RPM and divide by 5,252. For example, 350 lb-ft at 5,500 rpm is about 366.5 horsepower.
Why do torque and horsepower cross at 5,252 rpm?
Because horsepower equals torque times RPM divided by 5,252, the two numbers are equal at exactly 5,252 rpm when torque is measured in lb-ft.
How accurate is horsepower from quarter-mile time?
It is an estimate. Launch technique, traction and weather affect elapsed time a lot, so the trap speed formula is usually closer to the real wheel horsepower.
How many kW is one horsepower?
One mechanical horsepower is 0.7457 kilowatts. 300 hp is about 224 kW.
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Last updated October 2026.