Horsepower Calculator
Horsepower from torque and rpm, or estimated from weight with quarter-mile ET or trap speed, plus kW, PS and crank versus wheel horsepower after drivetrain loss.
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About this toolWhat is the Horsepower Calculator?
There are two ways to put a number on an engine's power without a brochure. If you have a torque reading from a dyno, horsepower follows exactly from torque and engine speed. If you have a timeslip from the drag strip, the car's weight and its quarter-mile elapsed time or trap speed give a surprisingly good estimate. This calculator does both, converts the answer to kilowatts and metric horsepower, and translates between flywheel and wheel figures with a drivetrain loss you can set.
With the defaults, 300 lb-ft at 6,000 rpm is 342.7 hp, 255.6 kW or 347.5 PS. Switch to the quarter-mile modes and a 3,500 lb car running 12.0 seconds points to about 400 hp at the crank by the Hale formula, while a 112 mph trap points to 384 hp. All three classic constant sets (Hale, Fox and Huntington) are available, with a side-by-side table so you can see how much the choice matters.
- Horsepower from torque in lb-ft or N·m and rpm, measured at the crank or the wheels
- Quarter-mile power estimates from ET or trap speed with Hale, Fox and Huntington constants
- Predicted ET and trap speed for a given power and weight
- Crank and wheel horsepower with drivetrain loss presets, plus kW and PS
- Side-by-side formula comparison, copy and share link
How to use the Horsepower Calculator
- Pick what you know: torque and rpm, quarter-mile ET, or trap speed.
- For torque, enter the value in lb-ft or N·m and the engine speed, and say whether it was measured at the crank or at the wheels.
- For a timeslip, enter race weight with the driver in lb or kg and the ET or trap speed, and choose a formula (Hale is the default).
- Pick a drivetrain type or type your own loss percentage.
- Read crank and wheel horsepower, kW and PS. With a weight entered, the calculator also predicts ET and trap speed for that power.
Worked examples
Torque at the crank
Input
300 lb-ft at 6,000 rpm, rear-wheel-drive manual (15%)
Output
342.7 hp, 255.6 kW, 347.5 PS; about 291 whp
At 5,252 rpm the same 300 lb-ft would be exactly 300 hp; above that speed horsepower is the larger number.
Metric torque reading
Input
400 N·m at 5,500 rpm
Output
308.9 hp, 230.4 kW
The calculator converts N·m to lb-ft (divide by 1.3558) before applying the 5,252 constant, or you can read kW directly.
Estimating from a timeslip
Input
3,500 lb with driver, 12.0 s ET
Output
Hale 400.3 hp, Fox 499.0 hp, Huntington 504.1 hp
The older constants assumed poorer traction, so they credit the same ET to much more power. Use Hale for modern cars on good tyres.
Wheel dyno to crank
Input
250 lb-ft at 5,000 rpm at the wheels, automatic (18%)
Output
238.0 whp, about 290.2 crank hp
Dividing by 0.82 gives 290 hp. Multiplying by 1.18 would give 281 hp, a common error that understates the engine.
Background and insight
Horsepower is torque times rotational speed. In US units hp = lb-ft x rpm / 5,252.11, where 5,252.11 is 33,000 / 2π: James Watt's 33,000 ft·lbf per minute divided by the radians in one revolution. That is why torque and horsepower curves always cross at 5,252 rpm. One mechanical horsepower is 745.7 W; the metric horsepower (PS, CV or ch) is 735.5 W, so a figure in PS is about 1.4% larger than the same power in hp.
The quarter-mile formulas are empirical fits that relate power-to-weight to elapsed time and trap speed with a cube-root law: ET = C x (weight / hp)^(1/3) and trap mph = C x (hp / weight)^(1/3). The constants commonly attributed to Huntington (6.290 and 224), Fox (6.269 and 230) and Hale (5.825 and 234) come from different eras of cars and tyres, and the power they assume is net engine power at the clutch, with weight including the driver. Trap speed tracks power more closely than ET because it is less sensitive to the launch.
A chassis dyno measures at the wheels, so the reading is lower than the engine's rating by the losses in the gearbox, driveshafts, differential and tyres. Typical rules of thumb are 10 to 15% for a manual front or rear-wheel-drive car, more for automatics and all-wheel drive. To go from wheel to crank divide by (1 − loss); multiplying by (1 + loss) understates the crank figure.
Common use cases
Reading dyno sheets
Turn a torque figure at a given rpm into horsepower, kW and PS, and estimate crank power from a chassis dyno run.
Drag strip estimates
Get a power figure from a timeslip when you have no dyno, or predict what ET and trap speed a power upgrade should bring.
Comparing spec sheets
Put European PS and kW ratings next to US horsepower on the same basis.
Power-to-weight planning
See how much power, or weight saved, it takes to reach a target quarter-mile time.
Tips and limitations
- Weigh the car with the driver and the fuel you race with. A 100 lb error moves the estimate by about 3%.
- Trap speed is the better input for power. ET is hurt by wheelspin, a slow launch or poor shifts that trap speed largely ignores.
- Quarter-mile formulas assume a typical car on a prepared surface. Altitude, heat, aerodynamics and gearing all shift the result, so compare runs from the same track.
- Drivetrain loss percentages are rules of thumb. Different chassis dynos read differently, and losses are partly fixed rather than a constant percentage.
- For the full torque, power and rpm relationship in other units, use the torque and power calculator.
Frequently asked questions
How do you calculate horsepower from torque and RPM?
Horsepower = torque (lb-ft) x rpm / 5,252. With 300 lb-ft at 6,000 rpm that is 1,800,000 / 5,252 = 342.7 hp. In SI units, power in kW = torque (N·m) x rpm / 9,549.
How do you estimate horsepower from quarter-mile trap speed?
Using Hale's constant, hp = weight (lb) x (trap mph / 234)³. A 3,750 lb car trapping 100 mph works out to about 293 hp at the crank.
What is the formula for horsepower from ET?
hp = weight x (5.825 / ET)³ with Hale's constant, where weight is in pounds including the driver. A 3,500 lb car running 12.0 seconds needs about 400 hp.
How much horsepower is lost through the drivetrain?
Rules of thumb are about 10 to 15% for manual front or rear-wheel-drive cars, 15 to 20% for automatics and around 20 to 25% for all-wheel drive. Real figures depend on the car and the dyno.
How do I convert wheel horsepower to crank horsepower?
Divide by one minus the loss fraction. With 340 whp and a 15% loss, crank power is 340 / 0.85 = 400 hp, not 340 x 1.15 = 391 hp.
What is the difference between hp, PS and kW?
One mechanical horsepower is 745.7 W; one PS (metric horsepower) is 735.5 W. So 100 hp = 74.57 kW = 101.4 PS. European makers quote PS or kW, US makers quote hp.
Are quarter-mile horsepower calculators accurate?
They are reasonable estimates for typical street and strip cars, often within about 10% when trap speed is used. Traction, aerodynamics, gearing and conditions can push them further off, so treat them as a guide, not a dyno.
Is there an in-depth guide to the Horsepower Calculator?
Yes. “Estimating Horsepower from Quarter-Mile ET and Trap Speed” covers the background, how it works, common pitfalls and worked examples. Read the full guide.
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