Engine Horsepower Calculator
Calculate engine horsepower using three proven methods: RPM & torque, quarter-mile elapsed time, or trap speed. Results display in HP, kW, and PS with full formula breakdown.
Estimated Horsepower
373.2 HP
Kilowatts
278.3 kW
Metric HP
378.4 PS
💡 HP = (RPM × Torque) ÷ 5,252 = (5,600 × 350) ÷ 5,252 = 373.2 HP
💡 How to Use the Engine Horsepower Calculator
How to Calculate Engine Horsepower
Horsepower (HP) is the standard unit of engine power output, invented by James Watt in the late 1700s to compare steam engine output to the work done by draft horses. One horsepower equals 33,000 foot-pounds of work per minute, or approximately 745.7 watts.
Method 1: RPM & Torque (Most Accurate)
This is the definitive formula. If you know your engine's peak torque (in lb-ft) and the RPM at which it occurs, this gives you the exact horsepower at that RPM. This is what a dynamometer measures — torque at every RPM — and then calculates HP from it.
Example: An engine produces 350 lb-ft of torque at 5,600 RPM: HP = (5,600 × 350) ÷ 5,252 = 373 HP.
Method 2: Quarter-Mile Elapsed Time
The Roger Huntington formula estimates horsepower from the quarter-mile elapsed time (ET) and vehicle weight. This method is popular at drag strips where a dyno isn't available. Weight includes the vehicle + driver + passengers.
Example: A 3,500 lb car runs a 13.0-second quarter mile: HP = 3,500 ÷ (13.0 ÷ 5.825)³ = 337 HP.
Method 3: Trap Speed
The trap speed method uses the speed recorded at the end of the quarter mile instead of elapsed time. This can be more reliable because trap speed is less affected by traction and reaction time.
Example: 3,500 lbs at 100 mph trap speed: HP = 3,500 × (100 ÷ 234)³ = 271 HP.
HP vs. kW vs. PS — What's the Difference?
| Unit | Full Name | Equivalent |
|---|---|---|
| HP | Mechanical Horsepower | 1 HP = 745.7 watts |
| kW | Kilowatt | 1 kW = 1.341 HP |
| PS | Pferdestärke (metric HP) | 1 PS = 0.9863 HP |
European manufacturers use kW or PS; American manufacturers use HP. The differences are minor but matter when comparing specs across markets.
Crank HP vs. Wheel HP
A dyno measures wheel horsepower (WHP) — the power that actually reaches the ground. Crank HP (what manufacturers advertise) is higher because the drivetrain loses 12–18% of power through the transmission and differential. Manual transmissions lose ~15%; automatics lose ~18%.