Engine Horsepower Calculator
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Results
About 316 hp at the crank for a 3,500 lb car trapping 105 mph with a 13.2 s ET
| Formula | From trap speed | From ET |
|---|---|---|
| Hale (234 and 5.825) | 316 hp | 301 hp |
| Fox (230 and 6.269) | 333 hp | 375 hp |
| Huntington (224 and 6.290) | 360 hp | 379 hp |
These are rough estimates from empirical formulas, not a dyno result. Trap speed depends less on the launch than ET does, so it is usually the steadier guide; a large gap between the two often points to traction or launch problems.
How to use the engine horsepower calculator
- Enter the race weight: the car with the driver and fuel as it ran, not the brochure curb weight.
- Enter the quarter-mile elapsed time, the trap speed, or both, from your time slip.
- Keep Hale's constants or compare Fox's and Huntington's; the table under the results shows all three.
- Adjust the drivetrain loss to see the matching wheel horsepower, or switch to the wheel ↔ crank mode to convert a dyno figure.
Quarter-mile horsepower formulas
From elapsed time: hp = weight ÷ (ET ÷ 5.825)³
Huntington: 224 and 6.29 · Fox: 230 and 6.269
Wheel hp = crank hp × (1 − drivetrain loss) · weight in lb, with driver
Both formulas say that speed and time depend on the cube root of the power-to-weight ratio. They were fitted to real drag strip results, not derived from first principles. Roger Huntington published the first widely used version in Rod & Custom magazine in December 1958, defining hp as peak net engine power at the clutch. Geoffrey Fox analyzed the physics in the American Journal of Physics in 1973, and drag racer and engineer Patrick Hale's 1986 Quarter programs used the 234 and 5.825 constants that most calculators quote today. These attributions follow Jeff Lucius's comparison of the formulas. Because they estimate engine power, multiply by one minus the drivetrain loss for the power at the wheels.
Worked example: a 3,500 lb car running 13.2 seconds at 105 mph
From the trap speed: 3,500 × (105 ÷ 234)³ ≈ 316 hp. From the ET: 3,500 ÷ (13.2 ÷ 5.825)³ ≈ 301 hp. The two agree within about 5%, which suggests a clean launch. With a 15% drivetrain loss, the trap-speed figure is about 269 hp at the wheels. Going the other way, a 255 hp chassis dyno reading at 15% loss suggests about 300 hp at the crank, and a 3,200 lb car with 400 hp should run about 11.65 s at 117 mph.
Estimated horsepower by ET and weight
Hale's formula, hp = weight ÷ (ET ÷ 5.825)³. Weight includes the driver.
| ET | 2,500 lb | 3,000 lb | 3,500 lb | 4,000 lb | 4,500 lb |
|---|---|---|---|---|---|
| 10.0 s | 494 hp | 593 hp | 692 hp | 791 hp | 889 hp |
| 11.0 s | 371 hp | 445 hp | 520 hp | 594 hp | 668 hp |
| 12.0 s | 286 hp | 343 hp | 400 hp | 458 hp | 515 hp |
| 13.0 s | 225 hp | 270 hp | 315 hp | 360 hp | 405 hp |
| 14.0 s | 180 hp | 216 hp | 252 hp | 288 hp | 324 hp |
| 15.0 s | 146 hp | 176 hp | 205 hp | 234 hp | 264 hp |
Estimated horsepower by trap speed and weight
Hale's formula, hp = weight × (mph ÷ 234)³.
| Trap speed | 2,500 lb | 3,000 lb | 3,500 lb | 4,000 lb | 4,500 lb |
|---|---|---|---|---|---|
| 90 mph | 142 hp | 171 hp | 199 hp | 228 hp | 256 hp |
| 100 mph | 195 hp | 234 hp | 273 hp | 312 hp | 351 hp |
| 110 mph | 260 hp | 312 hp | 364 hp | 416 hp | 467 hp |
| 120 mph | 337 hp | 405 hp | 472 hp | 539 hp | 607 hp |
| 130 mph | 429 hp | 514 hp | 600 hp | 686 hp | 772 hp |
| 140 mph | 535 hp | 642 hp | 750 hp | 857 hp | 964 hp |
Getting a better estimate
- Weigh the car. A public scale or the track's scale beats the brochure figure; every 100 lb changes the estimate for a 3,500 lb car by about 3%.
- Prefer trap speed. ET depends heavily on the launch, tyres and traction, while trap speed mostly reflects power and weight.
- Average several passes in similar weather. Hot, humid air or high altitude lowers the power an engine makes.
- Treat drivetrain loss as a rough guide. It varies with the transmission, drive layout and dyno, so a single percentage is a rule of thumb, not a measurement.
For torque, rpm and unit conversions, use the horsepower calculator. Only race on a sanctioned drag strip, never on public roads.
Frequently asked questions
How do you calculate horsepower from trap speed?
Use Hale's formula: hp = weight × (trap speed in mph ÷ 234)³, with the weight in pounds including the driver. A 3,500 lb car trapping 105 mph works out to 3,500 × (105 ÷ 234)³ ≈ 316 hp at the engine.
How do you calculate horsepower from quarter-mile ET?
Use hp = weight ÷ (ET ÷ 5.825)³. The same 3,500 lb car running 13.2 seconds gives 3,500 ÷ (13.2 ÷ 5.825)³ ≈ 301 hp. ET is more affected by the launch and traction than trap speed, so the two estimates rarely match exactly.
Do these formulas give wheel or crank horsepower?
They estimate engine (crank or flywheel) horsepower; Huntington defined it as peak net power at the clutch. For wheel horsepower, subtract the drivetrain loss: at 15%, 316 crank hp is about 269 hp at the wheels.
How much horsepower is lost through the drivetrain?
There is no single figure. Losses of roughly 10% to 20% are commonly assumed as a rule of thumb, depending on the transmission, drive layout and dyno. At 15%, a 255 wheel hp dyno reading suggests about 300 hp at the crank (255 ÷ 0.85).
What ET will a 400 hp car run?
It depends on weight. With Hale’s formulas, a 3,200 lb car (with driver) and 400 hp should run about 11.65 seconds at about 117 mph, assuming good traction. A heavier 4,000 lb car with the same power would be slower, at about 12.5 seconds.