Trap Speed & ET Horsepower Calculator

Enter your weight plus either a trap speed or an ET and this estimates horsepower using the two standard drag-racing regressions. Both are shown, because they often disagree — and that disagreement is informative. Treat the result as a range, not a number.

The two formulas

Both come from fitting curves to a lot of real timeslips, decades ago, and both survive because they work surprisingly well for ordinary cars. The trap-speed form scales with the cube of speed over weight; the ET form scales with the cube of the inverse of time. Neither knows anything about your engine — they only know how quickly a given mass got moving.

They are least accurate at the extremes: very light cars, very heavy cars, drag radials and slicks, aggressive aero, and anything that traps far outside the range the fits were built on. For a street car in the 10-15 second range they are a reasonable sanity check.

When the two numbers disagree

A gap between the trap-speed estimate and the ET estimate is a traction and launch story.ET estimate much lower than trap means the car left badly — wheelspin, bogging, or a slow 60-foot — and gave away time it had the power to keep. ET estimate higher than trapusually means a very strong launch on a short-geared car, or a shut-off before the lights.

Estimated is not measured

This page infers power from a result. It cannot see a power curve, so it cannot tell you where your torque lives, what a shift costs you, or whether you are down on power in one part of the rev range. That takes an actual measurement of acceleration through the run — which is what the app's dyno does from your phone's own telemetry.

Common questions

Why does this not match my dyno sheet?

Because they measure different things. A chassis dyno reads power at the wheels under controlled load; these formulas infer power from how fast a whole vehicle accelerated, so they bundle in drivetrain loss, tyre grip, aerodynamics, launch quality and driver skill. A 10-15% disagreement is completely normal, and the regressions also assume a competent launch.

Which is more reliable, the trap-speed method or the ET method?

Trap speed, generally. Trap speed depends mostly on power-to-weight and is fairly forgiving of a poor launch, while ET is heavily affected by how well the car left the line. If the two disagree a lot, the usual explanation is traction: a car that spins or bogs posts an ET much worse than its trap speed implies.

Is this crank horsepower or wheel horsepower?

These regressions were fitted to crank ratings, so treat the output as an approximate crank figure. Wheel horsepower is typically 10-15% lower for a manual and more for an automatic or all-wheel drive.

What weight should I enter?

Race weight — the car as it ran, including fuel and the driver. Using a dry curb weight from a spec sheet will understate the load the engine actually moved and inflate the estimate.