Engine Horsepower Calculator
0 estimated horsepower
Two Well-Established Estimation Formulas
Both formulas below are long-standing empirical models used by drag racers to back into an approximate flywheel horsepower figure using only a scale and a time slip — no dyno required. They trade precision for convenience, and each uses a different piece of quarter-mile data:
Trap-speed method: HP = weight (lb) × (trap speed (mph) / 234)3. This uses the speed your car was traveling at the end of the quarter-mile (the "trap").
Elapsed-time method: HP = weight (lb) / (ET (seconds) / 5.825)3. This uses the total time from launch to the finish line instead of trap speed.
Why the Two Methods Disagree
Both formulas were fitted decades ago to typical naturally-aspirated street/strip cars, so neither accounts for aerodynamic drag, tire compound, launch technique, altitude, or drivetrain losses. A car that leaves the line hard but trails off before the trap will show a lower ET-based estimate than trap-speed-based estimate, and vice versa for a car with a soft launch but a strong top end. Treat both results as ballpark flywheel horsepower, typically within 10-15% of a real dyno number, not an exact figure — use whichever data point you trust more, and consider both a plausible range rather than picking one as "correct."
Weight Matters as Much as the Time Slip
Because weight is a direct multiplier (or divisor) in both formulas, always weigh the car with a full tank and the driver aboard, not just the curb weight from the spec sheet — a 200 lb error in weight shifts the horsepower estimate by roughly the same percentage. If you're comparing multiple engine builds in the same chassis, pair this calculator with the speed calculator to sanity-check your trap speed and elapsed time against each other, or the weight calculator to convert between weight units before entering your vehicle's weight.
Frequently Asked Questions
Which method is more accurate, trap speed or elapsed time?
Neither is definitively more accurate — both are empirical approximations fitted to typical naturally-aspirated cars decades ago. Trap speed tends to better reflect top-end power, while elapsed time is more sensitive to launch and low-end torque. If you have both figures, calculate each and treat the pair as a plausible range rather than trusting one over the other.
Why does my calculated horsepower not match my car's dyno number?
These formulas estimate flywheel horsepower from weight and a single time-slip metric, ignoring aerodynamic drag, tire grip, altitude, temperature, and drivetrain losses. Dyno numbers (especially wheel horsepower) also measure differently. Expect these estimates to land within roughly 10-15% of a real dyno pull, not match it exactly.