Quarter Mile Calculator

It is an estimate, not a timing slip. The Fox and Hale equations tie power and weight to ET and trap speed and usually land within a few tenths for a well-sorted street car, but real results swing with traction, gearing, aerodynamics, altitude, transmission type, and driver skill. Use it to set expectations, not to settle a bet.

The classic equations were built around flywheel (crankshaft) horsepower, so enter that number. If your only figure comes from a chassis dyno, it is wheel horsepower, which is lower because of drivetrain loss. Add roughly 15 percent for a typical manual or automatic before entering it to approximate the flywheel value.

Enter race weight: the full curb weight of the car plus the driver, plus fuel and anything else on board during the pass. Heavier weight raises ET and lowers trap speed, so guessing low here is the most common reason an estimate looks quicker than the car really runs.

Trap speed is the cleaner read on power-to-weight because it depends far less on how well the car launches. Elapsed time is heavily shaped by the first sixty feet, so a car that spins the tires off the line can post a slow ET yet still trap a fast speed that matches its true power.

ET equals 6.290 times the cube root of weight divided by horsepower. Trap speed equals 224 times the cube root of horsepower divided by weight. Weight is in pounds and power is flywheel horsepower. These are the Fox and Hale forms, close cousins of the older Roger Huntington equations.

Not directly in this tool, which only goes from power and weight to a time. You can, however, try different horsepower values at your real weight until the predicted ET or trap speed matches a timing slip you already have, which brackets your actual output.

The equations assume a clean launch, good traction, and near sea-level air. Street tires, a high-altitude track, a slushy automatic, tall gearing, or a cautious launch all cost time the formula does not know about. Poor air density alone can add a tenth or more to ET on a hot day.

They are calibrated for typical gasoline cars, so treat other vehicles as rough guesses. Instant electric torque often beats the predicted ET, while a light motorcycle with a very high power-to-weight ratio falls outside the range the equations were fit to and can read off by more than the usual margin.

Yes. Every calculation runs locally in your browser, and none of the horsepower or weight figures you enter are sent to a server, so your numbers stay on your device.