Engine Horsepower Calculator
Estimate engine horsepower from quarter-mile elapsed time and vehicle weight.
Returns crank HP and estimated wheel HP using the ET drag racing method.
This calculator works backwards from a timeslip. Tell it what the car weighs with you sitting in it and what it ran over a quarter mile, and it estimates the horsepower needed to produce that run.
The formula (Huntington’s elapsed-time method): Horsepower = Weight ÷ (ET ÷ 5.825)³
Rearranged, it becomes the form most drag racers know: ET = 5.825 × (Weight ÷ Horsepower)^(1/3)
Roger Huntington fitted this in the 1950s from published quarter-mile times, and it has survived because the cube-root relationship holds up across wildly different cars. Look at what that cube root implies: to cut your ET by 10% you need roughly 37% more power. Power gains pay off slowly at the strip, which is why stripping weight out of the car is usually the cheaper way to find a tenth.
Crank or wheels? The formula was fitted against flywheel figures, so the headline number here is crank horsepower, comparable to a manufacturer’s brochure figure. The wheel horsepower line applies a 15% drivetrain loss, the usual rule of thumb for a manual rear-wheel-drive car. A torque-converter automatic loses more, often 18-22%, and an all-wheel-drive car more again.
Worked example: A 3,500 lb car, driver included, runs a 13.5 second quarter mile.
ET ÷ 5.825 = 13.5 ÷ 5.825 = 2.3176 2.3176³ = 12.45 HP = 3,500 ÷ 12.45 = 281 crank HP
After a 15% loss that is about 239 WHP, and the car is carrying 12.4 lbs for every horsepower.
Weigh the car with the driver in it. This is where most estimates go wrong. Curb weight off a spec sheet leaves out you, a half tank of fuel, and whatever has been living in the trunk. That is easily 250 lbs, worth about 20 HP on the estimate.
How this connects to torque. Horsepower and torque are linked through engine speed: Horsepower = (Torque × RPM) ÷ 5,252
The 5,252 is 33,000 ft-lbs per minute divided by 2π. One consequence is handy when reading a dyno chart: the two curves always cross at 5,252 RPM, because that is where the multiplier equals one.
Converting to kilowatts: kW = HP × 0.7457
Reference: typical outputs
- Economy car (1.5L): 110-130 HP, 16-17 second quarter mile
- Midsize sedan (2.5L): 180-205 HP, 15-16 seconds
- Performance V8: 400-500 HP, 12-13 seconds
- Heavy diesel truck: 400-600 HP but 900-1,800 ft-lbs, and a slow quarter. Torque moves loads, power moves the clock.
How we build and check this calculator
This calculator runs entirely in your browser, so the numbers you enter stay on your device. The math behind it is written by hand and tested against worked examples and standard references before the page goes live.
SuperGlobalCalculator is independently built and maintained. See how we build and verify our calculators.