Horsepower calculator: from torque and rpm or from a quarter-mile pass
This horsepower calculator works out engine power four ways. Enter torque and rpm to get horsepower exactly, the way a dyno does. Enter your car’s race weight and its quarter-mile trap speed or elapsed time (ET) to estimate the horsepower behind the run. Or go the other way: enter power and weight to estimate the ET and trap speed you could expect at the drag strip.
Every result is shown in mechanical horsepower (hp), metric horsepower (PS, the “cv” or “ch” on European spec sheets) and kilowatts, and an optional drivetrain-loss percentage converts between crank (flywheel) power and wheel power. The torque route is physics; the drag-strip routes use well-known empirical formulas and are labelled as rough estimates.
How to use the Horsepower Calculator
- Choose what to Calculate: HP from torque & rpm, HP from trap speed, HP from 1/4-mile ET, or ET & trap from HP.
- For torque, enter the torque in lb·ft, N·m or kgf·m and the rpm at which it was measured; say whether it was measured at the crank or at the wheels.
- For a drag-strip estimate, enter the race weight including the driver and the trap speed (mph or km/h) or ET, and pick a formula.
- Optionally type your drivetrain loss percentage to see crank and wheel power side by side.
- Read the result, the conversions and — for drag-strip modes — the same estimate from all three formulas, then copy it.
Horsepower Calculator features
Exact torque → power
P = τ × ω with exact unit factors, shown as the familiar hp = lb·ft × rpm ÷ 5,252 and kW = N·m × rpm ÷ 9,549.
Trap speed and ET estimates
Hale, Fox and Huntington formulas side by side, so you can see how much the answer depends on the method.
ET and trap speed from power
Predict a quarter-mile time from horsepower and weight before you go to the strip.
Crank vs wheel power
Enter your own drivetrain loss; the calculator never assumes a “standard” loss.
hp, PS and kW
Mechanical horsepower (745.700 W), metric horsepower (735.499 W) and kilowatts, plus lb or kg and mph or km/h inputs.
Horsepower from torque and rpm
Power is torque times rotational speed. In imperial units hp = torque (lb·ft) × rpm ÷ 5,252; the constant is 33,000 ÷ 2π, from James Watt’s definition of one horsepower as 33,000 foot-pounds per minute. In metric units kW = torque (N·m) × rpm ÷ 9,549. So 300 lb·ft at 5,000 rpm is 285.6 hp, and at 5,252 rpm torque and horsepower are numerically equal — which is why the two curves on any dyno chart in lb·ft and hp cross there.
Use the torque at the rpm you care about: peak horsepower usually comes well above the torque peak. One horsepower is 745.700 W; one PS (metric horsepower) is 735.499 W, so a 300 hp engine is 304 PS or 224 kW. For the general P = τω problem with any units, and for bolt tightening torque, see the torque calculator.
Quarter-mile formulas (rough estimates)
Drag racers have long estimated power from a timeslip with formulas of the form trap mph = C × (hp ÷ weight)1/3 and ET = k × (weight ÷ hp)1/3, with weight in pounds including the driver. Turned around, hp = weight × (mph ÷ C)3 and hp = weight × (k ÷ ET)3. Trap speed is the better guide to power, because ET depends heavily on the launch and traction.
The constants, as collected and compared on stealth316.com (checked October 2026): Patrick Hale’s QUARTER programs (1986) use C = 234 and k = 5.825; Geoffrey Fox’s 2001 fit uses 230 and 6.269; Roger Huntington’s 1958 Rod & Custom article “Horsepower at the Drag Strip” uses 224 and 6.290, with net horsepower at the clutch and the vehicle’s total weight with driver. The formulas differ by 10–15%, so treat the result as a ballpark figure, not a dyno reading.
| Example: 3,500 lb car | Hale | Fox | Huntington |
|---|---|---|---|
| Power from a 100 mph trap speed | 273.2 hp | 287.7 hp | 311.4 hp |
| Power from a 13.0 s ET | 314.9 hp | 392.5 hp | 396.5 hp |
| ET and trap speed with 300 hp | 13.21 s @ 103.2 mph | 14.22 s @ 101.4 mph | 14.27 s @ 98.8 mph |
Crank horsepower vs wheel horsepower
Manufacturers quote power at the crankshaft (flywheel), measured on an engine dyno. A chassis dyno measures power at the wheels, after the gearbox, driveshafts and differential have taken their share. The loss depends on the drivetrain, the dyno and how it is corrected, so this calculator asks for your own percentage instead of assuming one: wheel hp = crank hp × (1 − loss ÷ 100). If your dyno operator quotes a loss figure, use that. The drag-strip formulas expect crank power, so convert a wheel figure before estimating ET. For an engine’s displacement and bore/stroke, try the engine displacement calculator.
Horsepower Calculator FAQ
How do I calculate horsepower from torque and rpm?
Multiply torque in lb·ft by rpm and divide by 5,252. For example, 400 lb·ft at 4,500 rpm is 342.7 hp. In metric, kW = N·m × rpm ÷ 9,549.
How do I estimate horsepower from a quarter-mile trap speed?
Use hp = weight × (mph ÷ 234)³ with the race weight in pounds including the driver (Hale’s constant; Fox uses 230, Huntington 224). A 3,500 lb car trapping 100 mph has roughly 273–311 hp depending on the formula.
Is the quarter-mile horsepower result accurate?
Only roughly. The formulas are empirical fits to many cars and ignore traction, gearing, aerodynamics, altitude and weather. Use them to compare runs or sanity-check a dyno sheet, not to certify power.
What is the difference between hp, bhp and PS?
hp and bhp (brake horsepower) both mean mechanical horsepower measured at the crank, 745.7 W. PS is metric horsepower, 735.5 W, so a figure in PS is about 1.4% larger than the same power in hp.
How much power is lost through the drivetrain?
It depends on the car, the dyno and its correction method, so there is no single correct figure. Enter the loss your dyno operator or a back-to-back engine and chassis dyno test suggests.
Why do horsepower and torque cross at 5,252 rpm?
Because hp = lb·ft × rpm ÷ 5,252, the two numbers are equal at exactly 5,252 rpm when torque is in lb·ft and power in hp. With N·m and kW there is no such crossing at that speed.