Quarter mile calculator
Enter weight and horsepower, get the ET and trap speed.
How do you estimate a quarter-mile time from horsepower?
With Patrick Hale’s formulas from his 1986 drag-racing programs: ET = 5.825 × (weight ÷ hp)^⅓ and trap speed = 234 × (hp ÷ weight)^⅓, with the weight in pounds including the driver. They are empirical fits to real runs, not physics from first principles, so treat the answer as an estimate.
Example
A 3,200 lb car (with driver) making 400 hp has 8 lb per hp. The cube root of 8 is 2, so ET = 5.825 × 2 = 11.65 s and trap speed = 234 ÷ 2 = 117 mph. Roger Huntington’s older 1958 constants (6.290 and 224) give a slower 12.58 s at 112 mph.
Flywheel or wheel horsepower?
Flywheel (engine) horsepower — the rating that Huntington used and that Wallace Racing’s calculators ask for. A chassis dyno reads lower at the wheels because of drivetrain losses, so entering wheel horsepower underestimates the car.
Can I work out horsepower from my time slip?
Yes — turn Hale’s formulas around: hp = weight × (trap speed ÷ 234)³, or hp = weight × (5.825 ÷ ET)³. A 3,200 lb car trapping 117 mph works out to 400 hp. Trap speed is the better guide, because ET depends heavily on traction and the launch while trap speed mostly reflects power.
Why is my real time different?
The formulas assume a well-sorted car at sea level. Wheelspin, gearing, shift times, aerodynamics, altitude and weather all change real runs.
How much does 200 lb less weight help compared with 30 more hp?
For a 3,600 lb car with 300 hp, Hale’s formula gives 13.34 s at 102.2 mph. Removing 200 lb gives 13.08 s; adding 30 hp instead gives 12.92 s. The formula depends on weight ÷ power to the one-third power, so 10% more power helps about as much as 10% less weight.
Why a cube root
Power must overcome air drag, which grows with the cube of speed, and it must also accelerate the car's mass. Empirical fits to real drag-strip results therefore end up with the cube root of the weight-to-power ratio: ET = k × (weight ÷ hp)^⅓ and trap speed = m × (hp ÷ weight)^⅓. The constants differ by author, which is why the calculator offers three sets. A cube root has a modest effect: to cut the elapsed time by 10% you need roughly 37% more power at the same weight, because 1 ÷ 0.9³ is about 1.37.
A worked example with three formulas
Take a 3,600 lb car with driver and 300 hp, so 12 lb per hp, a cube root of 2.289. Hale gives 13.34 s at 102.2 mph. Huntington’s older constants give 14.40 s at 97.8 mph, and the Fox constants 14.35 s at 100.5 mph. Run the other way, the trap speed of 102.2 mph in a 3,600 lb car points back to 300 hp, which is the same figure you started with.
Common mistakes
Entering horsepower measured at the wheels into a formula built for flywheel horsepower underestimates the car, as the FAQ above explains. Leaving out the driver, fuel and cargo understates the weight; use the weight at the start line. Treating the result as a promise is another, since traction, launch technique, gearing, altitude and weather all move real times. And comparing an elapsed time from one track with a trap speed from another mixes the effects of traction and power; trap speed is the better indicator of power.
Sources: the quarter-mile formulas of Patrick Hale, Roger Huntington and the Fox constants as published by drag-racing calculators and used in the tests of this site; the cube-root relation is an empirical fit, not a physical law. Related: horsepower calculator, stopping distance calculator, acceleration calculator and engine displacement calculator.