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Quarter-Mile ET, Trap Speed & HP Calculator

Estimate quarter-mile ET, trap speed or flywheel horsepower from vehicle weight using the Roush/Fox drag-racing formulas.

Empirical estimate only — real ET, trap speed and horsepower depend on traction, gearing, aero, weather and driver. Not a dyno figure.

About this tool

Quarter-Mile ET, Trap Speed & HP Calculator is a free, in-browser tool that uses the well-known Roush/Fox drag-racing formulas to relate a car's weight, power and quarter-mile performance. Choose to predict elapsed time and trap speed from weight and horsepower, or to back out the horsepower implied by a weight and a measured ET or trap speed. Weight can be entered in pounds or kilograms and everything is computed locally in your browser.

The formulas are ET = 5.825 × (weight ÷ hp)^(1/3) for the quarter-mile elapsed time in seconds, and MPH = 234 × (hp ÷ weight)^(1/3) for the trap speed, with weight in pounds and hp at the flywheel. Solving for power inverts these: hp = weight ÷ (ET ÷ 5.825)³ from a known ET, or hp = weight × (MPH ÷ 234)³ from a known trap speed.

These are empirical estimates, not a substitute for a dyno or a real pass down the strip. Actual results vary with traction, gearing, aerodynamics, weather, driver skill and how much of the quoted weight is on track — treat the numbers as a ballpark, and use trap speed (which reflects power-to-weight) as the more reliable of the two.

Frequently asked questions

How accurate are these numbers?
They are estimates from empirical formulas fitted to typical cars. Real elapsed times and trap speeds shift with traction, gearing, aerodynamics, launch technique and weather, so treat the output as a ballpark rather than a guaranteed result.
Should I use trap speed or ET to estimate horsepower?
Trap speed is generally the better indicator of power-to-weight because it is less affected by launch and traction than elapsed time. If you have both, compare the two horsepower estimates — a big gap usually points to a traction or 60-foot problem.
Is the horsepower at the flywheel or the wheels?
The formulas are built around flywheel (crank) horsepower. Wheel horsepower measured on a chassis dyno is lower because of drivetrain losses, so it will read below the figure this tool estimates.
What weight should I enter?
The race weight — the car plus fuel and driver as it crosses the line. Using a dry curb weight without the driver will make the predictions optimistic. All math runs locally in your browser.

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