Trap Speed Horsepower Calculator
Trap speed, the speed your car reaches at the end of the quarter mile, is generally the most reliable single number for estimating power from a drag run. It reflects how the car accelerates over the whole distance rather than just the launch, so it is less sensitive to traction and driver reaction than elapsed time. This calculator applies the classic Hale trap speed relation to your race weight and trap speed to estimate horsepower at the wheels. Treat it as a sound field estimate, not a substitute for a dyno.
Trap speed horsepower formula
HP = weight * (trap speed / constant)^3
where weight is in pounds, trap speed in mph, constant defaults to 234
Power-to-weight = HP / weight
The constant 234 is the commonly published Hale value for power at the wheels. It is editable so you can match a value validated for your own combination.
Worked example
A 3,400 lb car trapping 100 mph: HP = 3,400 * (100 / 234)^3 = 3,400 * 0.078 = 265.34 horsepower. The power-to-weight ratio is 265.34 / 3,400 = 0.08 horsepower per pound.
Trap speed horsepower: frequently asked questions
Why is trap speed a good horsepower indicator?
Trap speed is the speed a vehicle reaches at the end of the quarter mile. Because it reflects sustained acceleration over the whole run rather than the launch alone, it correlates with power-to-weight ratio more reliably than elapsed time, which is heavily influenced by traction and reaction off the line. The Hale trap speed relation captures this correlation.
What is the trap speed formula?
The widely used Hale relation is horsepower equals weight times (trap speed / 234) cubed, with weight in pounds and trap speed in miles per hour. It is an empirical fit to drag strip data and estimates power at the wheels under good conditions, not exact crankshaft power.
Does air density or altitude affect the result?
Yes. The empirical constant assumes typical sea-level conditions. At high altitude or in hot, thin air, naturally aspirated engines make less power, and a given car will trap lower, which the relation reads as lower horsepower. For comparisons, run on similar days or apply a separate air-density correction.
Sources
- The Hale trap speed relation (constant 234 for power at the wheels) is a long-published empirical drag racing estimator, presented here as an engineering estimate rather than exact physics. Underlying SI units of mass, length and time follow NIST: NIST SI Units.
Reviewed by the CalculatorHub team, edited by James Graham, 19 June 2026. See our methodology.