Horsepower Calculator

Horsepower is power: how fast work is done. From an engine’s torque and speed, hp = torque (lb-ft) × RPM ÷ 5,252. An engine making 300 lb-ft at 5,000 RPM produces about 285.6 hp, or 213 kW.

How to use the calculator

1. Choose what you know: torque and RPM, power in kilowatts, or a car’s weight with its quarter-mile time or trap speed.
2. Enter the numbers and press Calculate. You get hp, kW, metric horsepower (PS) and watts.

Horsepower formulas

• hp = torque (lb-ft) × RPM ÷ 5,252. The 5,252 comes from 33,000 ft-lb per minute ÷ 2π. That is why torque and horsepower curves always cross at 5,252 RPM.
• 1 hp = 745.7 W and 1 kW = 1.341 hp.
• 1 metric horsepower (PS) = 735.5 W, slightly less than 1 hp.
• Quarter-mile estimates: hp ≈ weight × (5.825 ÷ ET)³ and hp ≈ weight × (trap mph ÷ 234)³.

Torque vs horsepower

Torque is twisting force; horsepower is how quickly that force does work. An engine with lots of torque at low RPM feels strong when pulling away or towing. An engine that keeps its torque at high RPM makes more horsepower and a higher top speed. Example: 400 lb-ft at 3,000 RPM is 228 hp, while 250 lb-ft at 7,000 RPM is 333 hp.

Horsepower practice questions

Test yourself with 6 multiple-choice questions. Choose an answer and press Submit to see if you are right and why. Your score appears at the end.

Frequently asked questions

How do you calculate horsepower from torque?

Multiply torque in lb-ft by RPM and divide by 5,252. With N·m, use kW = N·m × RPM ÷ 9,549 and then hp = kW × 1.341.

How many kW is 1 hp?

1 mechanical horsepower is 0.7457 kW. So 100 hp is 74.57 kW.

Why do horsepower and torque cross at 5,252 RPM?

Because hp = torque × RPM ÷ 5,252, the two numbers are equal at exactly 5,252 RPM when torque is in lb-ft.

Is wheel horsepower the same as engine horsepower?

No. Wheel horsepower is measured at the wheels and is lower, because the transmission and drivetrain lose some power, often 10% to 20%.

Sources

• NIST: Guide for the use of the SI, conversion factors
• SAE International: J1349 engine power test code

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