You enter the vehicle's weight in pounds and the quarter-mile elapsed time in seconds. The tool applies Fox's empirical equation — HP = Weight ÷ (ET ÷ 5.825)³ — to estimate engine horsepower. It then computes an estimated trap speed using 234 × (HP ÷ Weight)^(1/3) and a power-to-weight ratio in HP per U.S. ton. The formula assumes a standard drag strip at near sea level with no correction for altitude or weather.
Fox's equation is widely used as a quick estimate and is generally accurate within 5–10% for naturally aspirated vehicles on a prepared surface. Forced induction, nitrous, or poor traction can skew results significantly.
Use the weight of the vehicle as it crosses the finish line, including the driver and any fuel. For most street cars, curb weight plus 170–200 lbs for the driver is a reasonable approximation.
Dyno results measure power at the wheels, which is lower than flywheel HP due to drivetrain losses (typically 12–18% for RWD, 15–25% for AWD). Fox's equation estimates flywheel-equivalent output, so expect it to read higher than a wheel dyno number.
This calculator does not apply a density altitude correction. At higher elevations or in hot weather, engines produce less power, which increases ET. The HP number you see here reflects the conditions under which the run was made, not a corrected sea-level figure.
Estimate only. Results reflect your inputs and standard formulas. Double-check important decisions independently.