How this calculator works
The quarter mile formulas are empirical fits to drag strip results, not a physical simulation. Elapsed time depends strongly on traction and gearing in the first sixty feet, while trap speed depends mostly on power and weight, so the trap speed method is usually the better horsepower estimate. The elapsed time formula uses the common 5.825 constant and the trap speed formula uses 234, with weight in pounds, time in seconds and speed in miles per hour. Both methods estimate flywheel horsepower for a well-driven run at sea level and ignore air density, weather, tire slip and drivetrain losses.
Formula
From elapsed time: hp = weight(lb) / (ET / 5.825)^3. From trap speed: hp = weight(lb) x (speed(mph) / 234)^3. Weight to power = weight(lb) / hp.
Worked example
A 3,000 lb car running a 13.0 second quarter mile gives 3,000 / (13 / 5.825)^3 = 270 hp. The same car at 105 mph in the traps gives 3,000 x (105 / 234)^3 = 271 hp, so the two estimates agree closely.
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Frequently asked questions
Which method is more accurate?
Trap speed is generally more consistent because it is less sensitive to how well the car launches and shifts. Elapsed time changes a lot with traction, tire pressure and driver skill.
Is this wheel horsepower or flywheel horsepower?
The formulas are usually quoted as flywheel horsepower, because they were fitted to engine output. A chassis dyno reading at the wheels will be lower by the drivetrain loss.
Why does altitude change the result?
The formulas assume a standard air density. At high altitude the engine makes less power for the same tune, so a car runs slower than its sea-level horsepower would predict. Correct the result for density altitude if you compare runs at different tracks.
Can I use metric inputs?
Yes. Enter the weight in kilograms and the speed in km/h or m/s. The page converts to pounds and miles per hour before applying the formulas, and reports power in horsepower or kilowatts.