A dragstrip timing system is the only way to measure elapsed time and trap speed precisely, but two well-known empirical formulas let you estimate both from nothing more than horsepower and weight. This calculator implements the Hale formula for elapsed time and the Fox formula for trap speed, plus tools for comparing both at once or modeling a weight-reduction project before you start cutting.
How the Quarter-Mile ET Calculator works
The Hale formula, ET = 5.825 x (weight/hp)^(1/3), and the Fox formula, trap speed = 234 x (hp/weight)^(1/3), were both derived empirically from large samples of real drag-strip passes. They describe the same physical event from two angles — one predicting the finish-line clock time, the other the finish-line speed — so a car's ET and trap speed from these formulas should track each other closely. Both assume a reasonably clean launch and a naturally aspirated, street-tire pass; they are estimates, not substitutes for an actual timed run.
Inputs and what they mean
Horsepower should be the engine's output at the crank (flywheel), not wheel horsepower. Weight should be the vehicle as raced: curb weight plus driver and any extra ballast, entered in pounds. Because both formulas respond to the cube root of the power-to-weight ratio, large changes in horsepower or weight produce comparatively modest changes in ET and trap speed — which is exactly why the Weight Reduction Effect tab is useful for sanity-checking whether a diet project or an engine build is the better use of your budget.
Limits and edge cases
These formulas are most reliable for naturally aspirated cars on street tires making a clean, full-throttle pass; they get less accurate for cars with poor traction, all-wheel drive, forced induction, or heavy aerodynamic drag at speed. They also don't model launch technique, tire preparation, altitude, or NHRA class rules — treat the results as a ballpark estimate for planning purposes, not a guaranteed time slip. For a precise number, an actual timed pass at a sanctioned drag strip remains the gold standard.