Quarter Mile Calculator
Estimate quarter-mile and eighth-mile ET and trap speed from weight and horsepower, or work back to horsepower from a time slip — with Huntington, Fox and Hale formulas, wheel or crank power, driver weight and density altitude.
How it works
ET = k₁ × (weight ÷ hp)^(1/3) · Trap speed = k₂ × (hp ÷ weight)^(1/3) · Huntington k₁ = 6.290, k₂ = 224 · Fox 6.269, 230 · Hale 5.825, 234 · hp from ET = weight × (k₁ ÷ ET)³ · hp from trap = weight × (trap ÷ k₂)³ · eighth-mile ET ≈ ET ÷ 1.57, speed ≈ trap ÷ 1.26 (weight in lb, speed in mph)
Drag racers have long used simple power-to-weight rules to predict a quarter-mile time. Elapsed time (ET) is the seconds from the start line to the 1,320-foot mark, and trap speed is how fast the car is going as it crosses it. Both depend on the cube root of the weight per horsepower, which comes from physics: a car accelerating with constant power covers distance in proportion to time to the 1.5 power. Roger Huntington's 1950s constants are the classic choice; the later Fox and Hale versions predict slightly quicker times and higher speeds. You can also run the formulas backwards to estimate horsepower from a time slip: trap speed is the better guide, because it depends mostly on power, while ET also rewards a good launch. The formulas were calibrated on flywheel (crank) power and on race weight with the driver, so enter wheel horsepower as Wheel and add the driver's weight. Under More options you can set the drivetrain loss and the track's density altitude, which reduces the power a naturally aspirated engine makes. Eighth-mile estimates use the usual ratios of about 1.57 for time and 1.26 for speed. The what-if table shows how much power or weight it takes to drop a tenth.
Worked example
For a 3,300 lb car with 250 hp, the weight per horsepower is 13.2 lb/hp. Its cube root is 2.3633, so Huntington's ET is 6.290 × 2.3633 ≈ 14.87 s. The power-to-weight ratio 0.07576 has a cube root of 0.4231, giving a trap speed of 224 × 0.4231 ≈ 94.8 mph. The eighth mile would take about 14.87 ÷ 1.57 ≈ 9.47 s. Working backwards, a 3,750 lb car that runs 13.0 s has about 3,750 × (5.825 ÷ 13)³ ≈ 337 hp by Hale's formula, and one that traps at 100 mph has about 3,750 × (100 ÷ 234)³ ≈ 293 hp.
Sources: Omni Calculator — quarter-mile formulas (Huntington, Fox, Hale) · Wallace Racing — drag racing calculators
Frequently asked questions
Should I use crank or wheel horsepower?
The formulas were built around crank (flywheel) horsepower, the figure manufacturers quote. If you have a chassis-dyno number, choose Wheel and the calculator divides it by one minus the drivetrain loss (15% by default) to get the crank figure. When working back from a time slip, it shows both.
What is the difference between Huntington, Fox and Hale?
All three use the same cube-root form with different constants. Huntington's are the classic 1950s values; Fox's give slightly quicker times, and Hale's, from the 1980s, give the quickest times and highest speeds, which suits modern cars with sticky tires and quick-shifting transmissions. The formula table shows all three side by side.
Why does trap speed matter more than ET to some racers?
Trap speed depends mainly on power-to-weight and is less affected by how well you launch, so it is a cleaner measure of horsepower. ET rewards traction, the launch and the reaction of the transmission as well. That is why horsepower estimated from trap speed is usually more reliable than horsepower estimated from ET.
How do I convert an eighth-mile time to a quarter mile?
Multiply the eighth-mile ET by about 1.57 and the eighth-mile speed by about 1.26. A car accelerating with constant power would give 1.59 and 1.26 exactly; real cars spend part of the first eighth limited by traction, so the time ratio usually lands between 1.55 and 1.60.
How much does weight affect ET?
Weight and power count equally in the formula, so 1% less weight is worth about as much as 1% more power. On a 3,300 lb, 250 hp car the formula gives about 0.15 s for every 100 lb; the popular rule of thumb is a tenth per 100 lb. The what-if table works it out for your car.
How accurate are these estimates?
They are good ballparks for a well-sorted car with decent traction, often within a few tenths on ET and a few mph on trap speed. Poor traction, tall gearing, a slow shift or a heavy aero load can make a real ET noticeably slower than predicted.
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Educational estimate. Real-world results vary with conditions, tuning and measurement.