Engine Horsepower Calculator
Estimate engine horsepower from quarter mile ET or trap speed and race weight, or predict ET and trap speed from horsepower.
Drivetrain loss
A 3,500 lb car running 13.00 s needs about 315 hp at the crank.
Crank power split at your drivetrain loss.
Matching trap speed
The trap speed Hale's formula expects from this power and weight. Compare it with the speed on your time slip.
Your 105 mph trap points to 316 hp, 0.4% above the ET estimate. The two methods agree within 10%, which points to a clean pass.
Your calculation
The same steps the calculator runs, with your numbers.
- ET ratio: 5.825 ÷ 13 = 0.448077; cubed = 0.0899617.
- Horsepower: 3,500 × 0.0899617 = 314.87 hp at the crank.
- Matching trap: 234 × ∛(314.87 ÷ 3,500) = 104.85 mph.
- Trap check: 3,500 × (105 ÷ 234)³ = 316.22 hp.
- Wheels: 314.87 × (1 − 0.15) = 267.64 whp.
- Weight per hp: 3,500 ÷ 314.87 = 11.12 lb/hp.
- Power per ton: 314.87 ÷ (3,500 ÷ 2,000) = 179.9 hp/ton.
- Metric power: 314.87 × 0.7457 = 234.8 kW and 314.87 × 1.01387 = 319.2 PS.
ET vs horsepower curve
Quarter mile ET against crank horsepower for a 3,500 lb car. The dot marks this car.
Crank horsepower
ET and trap speed by horsepower at 3,500 lb
Hale's estimates at the same weight. The highlighted row is closest to this car.
| Crank hp | ET (s) | Trap (mph) | lb per hp |
|---|---|---|---|
| 225 | 14.54 | 93.7 | 15.56 |
| 250 | 14.04 | 97.1 | 14.00 |
| 275 | 13.60 | 100.2 | 12.73 |
| 300 | 13.21 | 103.2 | 11.67 |
| 325 | 12.86 | 106.0 | 10.77 |
| 350 | 12.55 | 108.6 | 10.00 |
| 375 | 12.26 | 111.1 | 9.33 |
| 400 | 12.00 | 113.6 | 8.75 |
| 425 | 11.76 | 115.9 | 8.24 |
| 450 | 11.54 | 118.1 | 7.78 |
Estimates from Patrick Hale's empirical formulas for cars near sea level. Race only at a sanctioned drag strip.
How the Engine Horsepower Calculator Works
This engine horsepower calculator estimates how much power a car makes from two numbers printed on a drag strip time slip: the quarter mile elapsed time (ET) or the trap speed, plus the race weight of the car. It also works in reverse. Enter horsepower and weight and it predicts the ET and trap speed the car should run, which helps when you plan an engine upgrade or a weight reduction.
The calculator uses the two short formulas published by drag racer and engineer Patrick Hale, the author of the Quarter and QUARTERjr simulation programs. Each formula ties power to weight through a cube or a cube root. Hale published the constants 5.825 and 234 as first order estimates in the tradition of earlier formulas fitted to real quarter mile results, so the answers are empirical estimates for typical street and strip cars at or near sea level, not readings from a dynamometer.
The default car, a 3,500 lb sedan that runs 13.0 seconds at 105 mph, gives 315 hp from the ET and 316 hp from the trap speed. When the two estimates land this close together, the pass was clean and the result is likely to be a fair estimate of the engine's output. The calculator compares the two automatically whenever both boxes hold a value.
Quarter Mile Horsepower Formulas: Hale, Fox and Huntington
Hale's equations are the latest of three well known versions. Roger Huntington published the first trap speed method in Rod & Custom magazine in December 1958, after about 40 to 50 road and strip tests in which he worked out the true peak horsepower delivered at the clutch with an accelerometer. Physics professor Geoffrey Fox described the theory behind the cube root relationship in the American Journal of Physics in 1973 and published updated constants in 2001. All three share the same shape and differ only in the constant.
| Method | Trap speed | ET | HP at 3,500 lb, 105 mph | HP at 3,500 lb, 13.0 s |
|---|---|---|---|---|
| Huntington | 224 × (hp ÷ wt)^⅓ | 6.290 × (wt ÷ hp)^⅓ | 360.5 | 396.5 |
| Fox | 230 × (hp ÷ wt)^⅓ | 6.269 × (wt ÷ hp)^⅓ | 333.0 | 392.5 |
| Hale (this calculator) | 234 × (hp ÷ wt)^⅓ | 5.825 × (wt ÷ hp)^⅓ | 316.2 | 314.9 |
Hale's trap constant is the largest of the three and his ET constant the smallest, so his formulas give the lowest horsepower for the same time slip. Jeff Lucius compared all three against 243 road tests published by Road & Track and found that Hale's formulas predict a lower ET and a slightly higher trap speed than typical modern street cars achieve. He suggests Hale's version suits race cars with better traction and drivetrains. For a street car on street tires, read the Hale result as the lower end of the likely range.
How to Calculate Horsepower From a Time Slip Manually
- Find the race weight. Weigh the car with the driver in the seat and the fuel it raced with, or add the driver's weight to the curb weight. Convert kilograms to pounds by dividing by 0.45359237.
- From ET: divide 5.825 by the ET, cube the answer, then multiply by the weight.
- From trap speed: divide the trap speed in mph by 234, cube the answer, then multiply by the weight. Convert km/h to mph by dividing by 1.609344.
- Compare. If the ET estimate sits well below the trap estimate, trust the trap figure more.
The same steps run in reverse to predict a time slip. A 3,200 lb car with 300 hp at the crank weighs 10.667 lb per hp. The cube root of 10.667 is 2.20128, so the predicted ET is 5.825 × 2.20128 = 12.82 s. The cube root of 300 ÷ 3,200 is 0.454280, so the predicted trap speed is 234 × 0.454280 = 106.30 mph.
How Much Horsepower to Run 10s, 11s, 12s and 13s
The table answers a common question: how much power does a car need for a given quarter mile time? Each value is the flywheel horsepower from Hale's ET formula at the race weight shown. Because ET × trap speed is fixed at 1,363.05 in Hale's pair, every ET also comes with one matching trap speed, whatever the weight.
| ET | 2,500 lb | 3,000 lb | 3,500 lb | 4,000 lb | Matching trap |
|---|---|---|---|---|---|
| 10.0 s | 494 hp | 593 hp | 692 hp | 791 hp | 136.3 mph |
| 11.0 s | 371 hp | 445 hp | 520 hp | 594 hp | 123.9 mph |
| 12.0 s | 286 hp | 343 hp | 400 hp | 458 hp | 113.6 mph |
| 13.0 s | 225 hp | 270 hp | 315 hp | 360 hp | 104.9 mph |
| 14.0 s | 180 hp | 216 hp | 252 hp | 288 hp | 97.4 mph |
The cube root makes each step faster cost more. Doubling the power of a car cuts its predicted ET by only about 21 percent, because the cube root of one half is 0.7937. Adding 10 percent more power raises the predicted trap speed by about 3.2 percent. Weight works the other way: taking 200 lb off the default 3,500 lb car at the same 315 hp drops the predicted ET from 13.00 s to 12.75 s.
These numbers assume the car launches and shifts well. In Lucius's comparison, typical street cars ran slower ETs than Hale predicts, so a street car on street tires usually needs more power than the table shows to reach the same time.
Why Trap Speed Beats ET for Estimating Power
Elapsed time measures the whole run from a standing start. It depends on the launch, wheelspin, the 60 foot time, gearing, shift speed and the condition of the track, and many of those have little to do with the engine. Trap speed is measured at the finish line, by which point the car has been accelerating on power for most of the run.
Fox wrote that weight and power have the largest influence on trap speed, while traction, aerodynamics, gearing and driver skill make ET harder to predict. Lucius measured the difference: against the Road & Track data, Hale's trap speed formula had a goodness of fit (R²) of 0.9124, while his ET formula reached only 0.6234. Huntington noted the same thing in 1976, writing that his ET plot had more scatter because traction varied between cars.
That is why the calculator shows a verdict badge when you enter both numbers:
- ET and trap agree means the two estimates are within 10 percent of each other.
- ET suggests launch losses means the ET estimate is more than 10 percent below the trap estimate. The engine is probably making the trap figure, and the car lost time at the start.
- Trap speed looks low means the ET estimate is more than 10 percent above the trap estimate, which is unusual. Lifting before the finish line or a wrong weight are common causes.
The 10 percent band is a practical threshold chosen for this calculator, not a published standard. It flags passes worth a second look.
Crank Horsepower, Wheel Horsepower and Drivetrain Loss
Crank horsepower, also called flywheel or brake horsepower, is the power measured at the engine's output. Manufacturers quote it, and engine dynamometers measure it. Wheel horsepower is what reaches the road after the transmission, driveshaft, differential, axles and tires take their share. A chassis dyno, where the car's driven wheels spin a roller, measures wheel horsepower.
The quarter mile formulas estimate crank horsepower. Huntington built his method from the net horsepower at the clutch, and Lucius tested all three formulas against manufacturer engine ratings, so the drivetrain loss is absorbed into the fitted constants. This calculator then applies your drivetrain loss percent to show wheel horsepower separately.
The default loss of 15 percent is a common rule of thumb, not a measured figure. Real losses depend on the drivetrain layout, the transmission type, the tires and the dyno itself, and the share is not constant across the rev range. If you have chassis dyno and engine dyno results for the same car, divide the wheel figure by the crank figure and subtract the answer from 1 to find your own loss. To turn a dyno torque reading at a given RPM into horsepower, or to work out electric motor horsepower, use the Horsepower Calculator.
Power to Weight Ratio
Every formula on this page depends on one number: the ratio of weight to power. Two cars with the same pounds per horsepower get the same predicted ET and trap speed, whatever their size. The calculator reports the ratio three ways so you can compare with figures from any market.
Lowering pounds per horsepower is the only way to move the predicted time slip, and you can do it by adding power or removing weight. For the default car, cutting 100 lb lowers the power needed for a 13.0 second pass from 314.87 hp to 305.87 hp, a saving of 9 hp.
Weather, Altitude and Dyno vs Drag Strip Numbers
An engine makes less power in thin, hot or humid air, so the same car runs slower at a high altitude track or on a summer afternoon than on a cool evening near sea level. Racers describe the air with density altitude, the altitude at which the standard atmosphere would have the same air density. This calculator does not correct for weather. Its results describe the car as it ran on the day, so compare passes made in similar conditions.
Dynamometer results are corrected to standard air. SAE J1349 is the SAE International test code for rating net engine power and torque, and the 2011 edition sets reference conditions of 25 °C inlet air and 99 kPa dry air pressure, with formulas that correct observed power to those conditions. The standard also states that its correction formulas are not intended for altitude derating. A corrected dyno figure and a strip estimate from a hot day can differ for this reason alone.
A dyno measures power directly at a steady or slowly rising engine speed. A time slip shows how the whole car used that power, including weight, traction, gearing and aerodynamics. Both are useful. Use the dyno for the engine and the strip for the car.
How to Use the Calculator
- Mode. Pick From ET, From trap speed, or Predict ET and trap.
- Units. Choose lb or kg for weight and mph or km/h for speed. Switching converts the value in the box.
- Weight. Enter the race weight with the driver, not the curb weight.
- ET and trap. Copy both from the time slip. The second one is optional and turns on the comparison badge.
- Horsepower. In predict mode, enter crank or wheel horsepower and choose which one it is.
- Drivetrain loss. Open the optional section to change the 15 percent default.
Common Mistakes
- Using curb weight. Leaving out a 200 lb driver on a 3,300 lb car understates power by about 6 percent.
- Using the 1/8 mile slip. These constants apply to the full quarter mile only.
- Entering km/h as mph. A 169 km/h trap read as mph more than quadruples the estimate.
- Treating the result as wheel horsepower. The formulas estimate crank power.
- Comparing passes from different weather. Hot, humid or high air lowers power.
Electric Cars, Heavy Trucks and Other Limits
The constants were fitted to cars with combustion engines and conventional gearboxes. Electric cars deliver full torque from a standstill and often launch harder than a gasoline car with the same peak power, so an ET based estimate for an EV can read high. The EV example, a 4,500 lb car running 11.5 seconds, gives 584.80 hp from the ET formula. Treat that as a rough comparison figure and use the trap speed method where you can.
Huntington tested cars with trap speeds between 70 and 115 mph and ETs no quicker than 12 seconds, and he extrapolated his curve above that range. Very light cars, heavy trucks, cars with unusual aerodynamics and dedicated drag cars on slicks all sit outside the data the formulas came from. The calculator rejects ETs outside 5 to 25 seconds and trap speeds outside 50 to 220 mph, because results that far out would be guesses.
These results are estimates for comparison and planning. Drag racing belongs at a sanctioned track with timing equipment and safety rules, not on public roads.
Frequently Asked Questions
How do you calculate horsepower from quarter mile time?
Divide the elapsed time (ET) by 5.825, cube the result, and divide the race weight in pounds by that number. Race weight means the car with the driver and fuel on board. For a 3,500 lb car that runs 13.0 seconds, 13 ÷ 5.825 = 2.2318, its cube is 11.116, and 3,500 ÷ 11.116 = 315 hp. This is Patrick Hale's empirical ET formula, and the answer estimates net engine horsepower at the flywheel. Traction, launch and shifting change ET a lot, so treat an ET based figure as a rough estimate and compare it with a trap speed estimate.
How do you calculate horsepower from trap speed?
Divide the quarter mile trap speed in mph by 234, cube the result, and multiply by the race weight in pounds, including the driver. A 3,500 lb car that traps 105 mph gives 105 ÷ 234 = 0.44872, cubed 0.09035, and 3,500 × 0.09035 = 316 hp. A 2,800 lb car trapping 98 mph works out to about 206 hp. The constant 234 comes from Patrick Hale, and the result estimates flywheel horsepower. Trap speed depends mostly on power and weight, so it usually gives a steadier estimate than elapsed time.
How much horsepower do I need to run an 11 second quarter mile?
Using Hale's ET formula, the flywheel horsepower needed for an 11.0 second pass is about 445 hp at 3,000 lb, 520 hp at 3,500 lb and 594 hp at 4,000 lb race weight. A 10.0 second pass needs about 593 hp at 3,000 lb and 692 hp at 3,500 lb, while a 12.0 second pass needs about 343 hp at 3,000 lb and 400 hp at 3,500 lb. These figures assume a car that launches and shifts well. A street car on street tires usually needs more power than the formula suggests, because some time is lost at the start.
Is trap speed or ET better for estimating horsepower?
Trap speed is usually the better guide. Elapsed time includes the launch, wheelspin, gear changes and the reaction of the tires to the track, while trap speed depends mainly on power and weight. In Jeff Lucius's comparison with 243 Road & Track road tests, Hale's trap speed formula had a goodness of fit (R²) of 0.9124, but his ET formula reached only 0.6234. If the ET estimate is far below the trap estimate, the car probably lost time at the start rather than lacking power.
Do quarter mile formulas give wheel or crank horsepower?
They estimate engine horsepower at the flywheel, often called crank horsepower. Roger Huntington built the original method from the net horsepower delivered at the clutch, and Jeff Lucius tested the Hale, Fox and Huntington formulas against manufacturer engine ratings. Drivetrain losses are absorbed into the fitted constants rather than entered separately. To estimate wheel horsepower, multiply the crank figure by one minus your drivetrain loss: 316 hp × (1 − 0.15) = 269 whp. To go back from a chassis dyno reading, divide instead: 300 whp ÷ 0.85 = 353 hp at the crank.
How accurate is the quarter mile horsepower formula?
It is a first order estimate, not a measurement. The constants were fitted to real cars, and Lucius found that Hale's formulas predict a lower ET and a slightly higher trap speed than typical modern street cars achieve, so they suit well prepared drag cars with good traction better. Results also change with weather, altitude, aerodynamics, gearing and tires. Use the estimate to compare runs of the same car or to sanity check a dyno figure, and treat a dyno test corrected to SAE J1349 conditions as the more precise number.
Sources & References
- Lucius, J. Calculators for 1/4 Mile ET and MPH vs. HP and Weight, Stealth 316 (history and constants of the Huntington, Fox and Hale formulas, Huntington's 1976 letter, and a comparison with 243 Road & Track road tests). stealth316.com
- Fox, G. T. On the Physics of Drag Racing, American Journal of Physics 41, 311 to 313, March 1973. doi.org
- Wallace Racing. ET and MPH from HP and Weight Calculator, Patrick Hale's Formulas. wallaceracing.com
- National Institute of Standards and Technology. NIST Guide to the SI (Special Publication 811), Appendix B.8: Factors for Units Listed Alphabetically (horsepower of 550 ft·lbf/s = 745.6999 W, metric horsepower = 735.4988 W, pound = 0.4535924 kg). nist.gov
- SAE International. J1349 Engine Power Test Code: Spark Ignition and Compression Ignition, As Installed Net Power and Torque Rating, revised November 2025. saemobilus.sae.org
- SAE International. SAE J1349, revised September 2011, reading room copy (reference atmospheric conditions and correction formulas). sae.org
Results are estimates based on empirical formulas and the values you enter, not dynamometer measurements. Every number on this page was recomputed with the same formulas the calculator uses. Methodology reviewed: 2026.