Engine Horsepower Calculator
Estimate a car's quarter-mile elapsed time and trap speed from its weight and engine power, or work back from a time slip to engine horsepower. The headline figure uses Patrick Hale's formula, with Geoffrey Fox's and Roger Huntington's beside it.
Elapsed time, Hale's formula
12.444 s
Trap speed, Hale's formula
109.53 mph
| Formula | Elapsed time (s) | Trap speed (mph) |
|---|---|---|
| Hale | 12.444 | 109.53 |
| Fox (2001) | 13.393 | 107.66 |
| Huntington | 13.438 | 104.85 |
Each figure is an empirical estimate for a car in good traction conditions, not a measurement.
The three formulas are Patrick Hale's, Geoffrey Fox's 2001 pair and Roger Huntington's, as printed on Formulas for 1/4 mile ET & mph vs. hp & wgt (Stealth 316). For Huntington's formula that page defines the weight as the total weight of the vehicle with driver, in pounds, and the power as peak engine horsepower output at the clutch, net power. That is engine power, not wheel horsepower. Its worked table for Hale and Fox uses a 3,900 lb test weight with driver. We opened it on 2026-10-15.
The formulas work in pounds and horsepower. One pound is exactly 0.45359237 kg, from the NIST Guide to the SI, footnotes to Appendix B.NIST Guide to the SI, Appendix B.8 defines one horsepower as 550 ft·lbf/s, which is 745.69987158227022 W. One PS is 75 m·kp/s, or 735.49875 W, from Die gesetzlichen Einheiten in Deutschland (PTB).
How to use the Engine Horsepower Calculator
- Pick what to estimate: ET and trap speed from power and weight, horsepower from weight and trap speed, or horsepower from weight and elapsed time.
- Enter the weight of the car with the driver in it, in pounds or kilograms.
- Enter the engine power in hp, kW or PS, the trap speed in mph, or the elapsed time in seconds. In the two horsepower modes, pick the unit you want the answer in.
- The results update as you type, so there is no button to press.
The quarter-mile formulas
Trap speed = C × (hp ÷ weight)^(1/3) · · · ET = K × (weight ÷ hp)^(1/3) · · · Hale: C = 234, K = 5.825 · · · Fox (2001): C = 230, K = 6.269 · · · Huntington: C = 224, K = 6.290
Weight is in pounds and power in horsepower. Turned around, the same formulas give horsepower = weight × (trap speed ÷ C)³ or weight × (K ÷ ET)³. All three pairs come from Jeff Lucius's Stealth 316 page. It defines Huntington's inputs as the total weight of the vehicle with driver and peak engine horsepower at the clutch, net power, so the answer is engine power, not wheel horsepower. Each result is an empirical estimate for a car in good traction conditions, not a measurement.
Worked example
Stealth 316 runs the numbers for a 3,900 lb car, driver included, at 400 hp. Hale's formula gives 5.825 × (3,900 ÷ 400)^(1/3) = 12.444 s and 234 × (400 ÷ 3,900)^(1/3) = 109.53 mph. Fox's gives 13.393 s at 107.66 mph, and Huntington's 13.438 s at 104.85 mph. The Hale and Fox figures match the table on that page. Going the other way, 3,900 lb at 109.53 mph comes to 400.0 hp by Hale's formula, 421.2 hp by Fox's and 456.0 hp by Huntington's.
Frequently asked questions
Which of the three formulas should I trust?
Stealth 316 checked them against Road & Track tests of 243 late-model cars. Hale's formulas predicted a much lower ET and a bit faster trap speed than is typical for modern street cars, and the page suggests they may suit race-track cars with better traction. For trap speed all three fit the data reasonably, with Hale's matching the faster cars and Huntington's the slower ones.
Why do ET and trap speed give different horsepower?
Stealth 316 says the trap speed formula can be more accurate for horsepower, because traction, launch technique, gear ratios and shifting affect it less. Huntington saw the same in his own tests. His elapsed times scattered more, because traction varied from car to car.
Does the weight include the driver, and is the power at the wheels?
The weight includes the driver. Stealth 316 defines it as the total weight of the vehicle with driver, in pounds. The power is peak engine horsepower at the clutch, net power, so the calculator expects engine power, not wheel horsepower.
What range were the formulas tested on?
Huntington wrote that he only tested cars with trap speeds between 70 and 115 mph, and that his curve is extrapolated above 115 mph. His quickest tested elapsed time was 12 seconds. The calculator still answers outside that range, and those figures are extrapolation.