This horsepower-from-ET calculator estimates your engine's power from drag-strip time-slip data — either your quarter-mile elapsed time (ET) or your trap speed (MPH), combined with race weight. It's the go-to way to gauge horsepower when you don't have access to a dyno.
Horsepower from ET & Trap Speed Formula
From ET: HP = Weight × (6.290 ÷ ET)³
These are the inverse of the classic quarter-mile prediction formulas developed by Roger Huntington and refined by Patrick Hale. Because performance scales with the cube root of power-to-weight, horsepower scales with the cube of speed — which is why the formulas raise the speed (or 6.290 ÷ ET) ratio to the third power. The result is flywheel (crank) horsepower, not wheel horsepower.
Why Trap Speed Beats ET for Estimating Power
Trap speed is measured at the finish line and reflects the total energy the car accumulated over the run, so it's a clean proxy for engine power. Elapsed time includes everything that happened off the line — a bad 60-foot, wheelspin, a slow reaction, tall gearing — none of which is about engine output. Two cars with identical horsepower can post very different ETs but nearly the same trap speed. If your ET-based and trap-based estimates disagree, trust the trap number and look at your launch.
What Affects Accuracy
These empirical formulas assume reasonably clean conditions. Aerodynamics throw them off at the extremes — very slippery or very draggy cars deviate from the 234 constant. Density altitude (hot, humid, high-elevation air) lowers both real power and trap speed together, so the estimate stays roughly consistent. For the most trustworthy number, use trap speed from a clean pass on a well-prepped track, and remember the output is flywheel HP — subtract drivetrain loss with our wheel horsepower calculator to compare against a chassis dyno.
Estimated HP by Trap Speed (3,200 lb car)
| Trap Speed (MPH) | Est. Flywheel HP | Power-to-Weight |
|---|---|---|
| 100 | 250 | 0.078 hp/lb |
| 110 | 333 | 0.104 hp/lb |
| 115 | 380 | 0.119 hp/lb |
| 125 | 488 | 0.152 hp/lb |
| 140 | 685 | 0.214 hp/lb |
Worked Example
This calculator provides estimates based on standard mathematical formulas. Real-world results will vary based on mechanical condition, environmental factors, and other variables.
Frequently Asked Questions
Trap speed is much better. A bad launch will ruin ET but won't reduce trap speed much, meaning trap speed provides a more consistent HP calculation.
ET-based estimates are typically within 5–10% for a well-prepared car with good traction. Poor launches, wheelspin, or a heavy track surface make the real horsepower higher than the estimate suggests.
Always use race weight: the car as it runs down the track, including fuel and driver. Leaving the driver out understates weight and underestimates horsepower.
Trap speed reflects sustained power output, while ET is heavily affected by the launch and 60-foot time. Two cars with the same ET but different traction can have quite different horsepower.
Yes. The formula depends only on weight and elapsed time, so it works for any vehicle, though very heavy or aerodynamically unusual vehicles will show more error.
The 60-foot time measures launch quality. A poor 60-foot adds to ET without reflecting engine power, so a car that launches badly will look down on horsepower in an ET-based estimate.