Horsepower Calculator
Estimate your engine’s horsepower from a quarter-mile run. Enter the vehicle weight (with driver) and either the trap speed or the elapsed time to apply the classic drag-strip formulas — then convert between hp, kW, PS, and watts.
Trap speed is the speed at the quarter-mile mark, not the average. Weigh the car as raced — driver, fuel, and gear included.
How a quarter-mile run reveals horsepower
Accelerating a mass down a quarter mile takes power, so weight plus either finishing speed or elapsed time pins down the engine’s output. The empirical formulas — popularized by automotive engineer Roger Huntington from decades of drag-strip data — assume a full-effort straight-line run: Trap-speed: HP = weight × (speed ÷ 234)³. ET: HP = weight ÷ (ET ÷ 5.825)³.
The trap-speed method is usually the more reliable of the two because launch technique, traction, and reaction time barely affect the speed you carry through the traps, while they heavily distort the elapsed time. Both estimate power at the flywheel for a conventionally geared car; all-wheel-drive launches and EV torque delivery bend the assumptions.
How it’s calculated
Trap-speed: HP = W × (S/234)³ with W in lb and S in mph. ET: HP = W ÷ (t/5.825)³. Metric inputs convert first (1 kg = 2.2046 lb; 1 km/h = 0.6214 mph). Power conversions: 1 hp = 745.700 W; 1 PS = 735.499 W; 1 kW = 1.341 hp.
Empirical street/strip estimates — traction limits, altitude, weather, and drivetrain type can move real dyno numbers meaningfully. Never measure times on public roads.
Worked example
A 3,400 lb car (driver in) trapping 108 mph works out to 334 hp (249 kW / 339 PS) — about 10.2 lb per horsepower. If the same car runs a 12.5-second ET, the ET formula suggests 344 hp; when the two disagree, trust the trap-speed figure and look for a traction or launch explanation.
Common mistakes
- Using average speed instead of the speed at the quarter-mile traps.
- Weighing the car empty — the driver and half a tank add 200+ lb.
- Taking the ET method at face value with wheelspin or a bad launch in the run.
- Confusing PS/CV/ch (metric horsepower, 735.5 W) with SAE hp (745.7 W) — European brochures often quote PS.
Where it is used
- Estimating power gains after intake, exhaust, or tune changes from strip slips.
- Sanity-checking a seller’s horsepower claims against their timeslip.
- Converting between hp, kW, and PS across US and European spec sheets.
- Bench-racing: predicting trap speed from a target power-to-weight.
Frequently asked questions
Which method is more accurate?
Trap speed. It reflects the power that accelerated the car through the whole run and barely cares about the launch, while ET is very sensitive to traction and driver skill. Racers use ET for bracket consistency but trap speed for power estimates.
Is this wheel horsepower or crank horsepower?
The formulas were calibrated to roughly match flywheel (crank) horsepower for conventional rear-drive cars. A chassis dyno reads lower — drivetrain losses run about 10–15% for manual RWD and more for AWD.
What are hp, PS, and kW exactly?
Mechanical hp = 550 ft·lb/s = 745.7 W (James Watt’s definition). PS (Pferdestärke, also CV/ch) = 735.5 W, about 1.4% smaller — common in European specs. The SI unit is the watt: 100 kW = 134.1 hp = 136.0 PS.
Can I use km/h and kilograms?
Yes — set the unit selectors and everything converts internally. A 1,540 kg car trapping 174 km/h is the same run as 3,395 lb at 108 mph.
Why does my result differ from the dyno?
Altitude and weather (density altitude), tire spin, shift quality, and aero drag all move strip numbers, and dynos themselves vary by type and correction factor. Treat agreement within ±5% as excellent.