Horsepower from Trap Speed Calculator
Estimate engine horsepower from your quarter-mile trap speed and vehicle weight using Hale’s trap-speed formula.
How it works
Horsepower = weight × (trap speed ÷ 234)³ (Hale formula) · weight in lb, trap speed in mph
Trap speed — how fast you are going as you cross the quarter-mile line — reflects power far better than elapsed time, because it depends on average power relative to weight rather than launch traction. Patrick Hale’s widely used formula backs horsepower out of trap speed and weight, with the constant 234 folding in typical drivetrain and aerodynamic losses. It estimates flywheel power, so wheel horsepower will read lower by the driveline loss.
Worked example
A 3,300 lb car (with driver) trapping 100 mph gives 3,300 × (100 ÷ 234)³ = 3,300 × 0.0781 ≈ 258 horsepower — a power-to-weight of about 12.8 pounds per horsepower.
Sources: Quarter mile — trap speed estimation
Frequently asked questions
Why estimate power from trap speed instead of ET?
Elapsed time is heavily affected by the launch — traction, reaction and 60-foot time — which can mask the engine’s true output. Trap speed depends mainly on average power and weight over the whole run, so it correlates far better with horsepower.
Is this flywheel or wheel horsepower?
Hale’s formula estimates flywheel (crank) horsepower, since the constant is calibrated to typical whole-car losses. Wheel horsepower measured on a dyno is usually 10–20% lower because of drivetrain friction between the crank and the tires.
How accurate is the trap-speed method?
For a reasonably conventional car it is typically within about 5–10%. It assumes average aerodynamics and drivetrain losses, so unusually slippery or draggy vehicles, big wings or heavy driveline losses will shift the estimate. Always include the driver’s weight.
Related calculators
Educational estimate — real-world figures vary with load, temperature, traction, tuning and measurement method.