How to use the engine horsepower calculator
- Choose whether to estimate from the elapsed time (ET) or the trap speed on your time slip.
- Enter the vehicle weight with the driver in pounds or kilograms. Use the race weight, including fuel and anything in the car.
- Enter the quarter-mile ET in seconds, or the trap speed in mph or km/h.
- Pick the formula. Hale's is the most widely used; Fox and Huntington are shown for comparison.
- Enter a drivetrain loss to estimate horsepower at the wheels.
- Read the flywheel estimate, the wheel estimate and the table comparing all three formulas.
The formulas
These are empirical formulas: constants fitted to many drag strip results from cars whose power was known. They share one idea. If an engine delivers a roughly steady power over a fixed distance, the speed at the end grows with the cube root of power-to-weight, and the time shrinks with it.
Hale's constants come from turning his forward predictions, ET = 5.825 × (weight ÷ hp)^⅓ and MPH = 234 × (hp ÷ weight)^⅓, around to solve for horsepower. The result approximates the engine's rated (flywheel, or crank) horsepower, because the formulas were fitted to manufacturer ratings.
Worked example
The defaults are a 3,500 lb car, driver included, running a 13.5-second quarter mile, with Hale's formula and 15% drivetrain loss:
- ET ÷ 5.825 = 13.5 ÷ 5.825 = 2.3176
- Cubed: 2.3176³ = 12.448
- Flywheel horsepower: 3,500 ÷ 12.448 = 281 hp (210 kW)
- At the wheels: 281 × (1 − 0.15) = 239 hp
- Trap speed that power predicts: 234 × (281.2 ÷ 3,500)^⅓ = 101.0 mph
If the same car's slip shows a 105 mph trap speed, the trap speed method gives 3,500 × (105 ÷ 234)³ = 316 hp. With Fox's constant the ET method gives 350 hp, and with Huntington's 354 hp. The spread shows why these are estimates, not dyno numbers.
ET or trap speed: which to trust?
| Factor | Effect on ET | Effect on trap speed |
|---|---|---|
| Wheelspin or a poor launch | Much slower | Little change |
| Short gearing, quick shifts | Faster | Small effect |
| Hot, humid or high-altitude air | Slower | Slower |
| Aerodynamic drag at high speed | Slower | Slower |
Elapsed time depends heavily on the launch, so a car that spins its tires posts a slow ET and the ET method underestimates power. Trap speed reflects the power over the whole run, so most racers treat it as the better estimate. When the two methods disagree widely, as in the example, the launch is usually the reason.
Flywheel vs. wheel horsepower
Flywheel horsepower is what the engine makes at the crankshaft, the figure carmakers quote. Wheel horsepower is what reaches the tires after the transmission, driveshaft and differential take their share. A chassis dyno measures wheel horsepower. Common rules of thumb put the loss at about 10 to 15% for a manual gearbox and 15 to 20% for an automatic or all-wheel drive. Use the horsepower calculator if you have torque and rpm figures instead.
Frequently asked questions
How accurate are horsepower estimates from quarter-mile times?
Expect them to be within about 10% for a typical street car with a clean run. Traction, gearing, tire size, weather and track elevation all move the numbers. Use the same formula each time to compare runs or modifications.
What weight should I enter?
Enter the race weight: the car with fuel, the driver and anything else on board. Leaving out a 180 lb driver from a 3,500 lb car lowers the estimate by about 5%.
Why does the ET method give lower horsepower than trap speed?
Usually because of the launch. Wheelspin, a slow reaction from the transmission or a soft launch add time early in the run without changing the trap speed much, so the ET method reads low.
Is the result flywheel or wheel horsepower?
The formula result is closer to flywheel horsepower, the engine's rated output. The calculator subtracts the drivetrain loss you enter to estimate wheel horsepower, which is what a chassis dyno would show.
Related calculators
- Horsepower Calculator — Horsepower from torque and rpm, or from force, distance and time, plus hp, PS and kW conversions.
- Speed Calculator — Speed from distance and time, distance from speed and time, or time from distance and speed, in any unit.
- Weight Calculator — Weight force W = m × g on Earth, the Moon and the planets, in newtons, pound-force and kilogram-force.
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