Quarter-Mile ET and Trap-Speed Calculator

Background.

How quick a car runs the quarter mile comes down, to a first approximation, to one ratio: how many pounds each horsepower has to move. A 3,200 lb car with 400 hp carries 8 lb per horsepower, and on the Fox street-car constants that predicts 12.538 seconds at 115 mph. The relationship runs through a cube root, which is why big changes buy surprisingly little: double the power to 800 hp and the estimate improves only to 9.951 seconds, about a fifth quicker rather than twice as quick.

Three constant sets are offered. Huntington (6.290 for time, 224 for speed) and Fox (6.269 and 230) describe street cars. Hale (5.825 and 234) describes race-prepared cars that launch harder and shift faster, so it predicts a quicker run from identical numbers: 11.65 seconds for the same 400 hp car, close to nine-tenths quicker than Fox. That spread is a fair picture of what preparation can be worth next to the specification sheet.

No power-to-weight formula sees what happens where the tires meet the track. Two cars with the same weight and power can be a second apart if one spins off the line, has tall gearing or a slow-shifting automatic, or runs on a hot afternoon at altitude. Trap speed usually follows power more faithfully than elapsed time, because the speed at the far end depends less on how the first sixty feet went, and that difference turns out to be useful. It lets the estimate do two jobs well: comparing changes on one car, such as what 150 lb of weight saving or a 50 hp gain might be worth, and reading a real time slip to tell a launch problem from a shortage of power.

What is quarter-mile calculator?

A quarter mile is 1,320 feet, exactly 402.336 metres, measured from the starting line to the finish line of a drag strip. Elapsed time, or ET, is the time the car takes to cover that distance: the clock starts when the front tires leave the starting beam and stops when the car breaks the finish beam, so the driver's reaction to the lights is timed separately and is not part of it. Trap speed is the speed the car carries through a short timed section just before the finish line, an average over that last stretch rather than a reading at a single point. A time slip also shows intermediate splits, most importantly the 60-foot time, which reflects the launch almost on its own. Weight per horsepower is the ratio that links them all here: the lower it is, the quicker and faster the car is predicted to be.

How to use this calculator.

1. 01Enter total race weight in pounds as the car will actually run: driver in the seat, the fuel load you race with, and anything bolted in or carried. A public weighbridge or the track's scales give a real figure; the brochure curb weight leaves out the driver. From kilograms, multiply by 2.2046, so 1,450 kg is about 3,197 lb.
2. 02Enter horsepower. A figure in kilowatts converts by multiplying by 1.341 (220 kW is about 295 hp), and metric PS by multiplying by 0.9863. Decide whether you are using the maker's engine rating or a chassis-dyno figure measured at the wheels, and keep to that choice for every comparison.
3. 03Choose the model. Fox or Huntington suit a street car on street tires; Hale suits a car set up to race, with sticky tires, a launch-friendly clutch or converter and quick shifts.
4. 04Read the ET, the trap speed and the weight per horsepower. To weigh up a modification, change only the weight or the power and leave the model where it was, since switching models moves the answer more than most upgrades do.
5. 05To check a real pass, enter the weight from the scales and the power you believe the car makes. A predicted trap speed well above what you ran suggests the power figure is optimistic; a trap speed that matches while the ET lags points to time lost at the launch.

The formula.

The two estimates are ET = C_ET × (W ÷ HP)^(1/3) and MPH = C_MPH × (HP ÷ W)^(1/3), where W is race weight in pounds, HP is horsepower and the constants come from the chosen model: 6.269 and 230 for Fox, 6.290 and 224 for Huntington, 5.825 and 234 for Hale.

The cube roots are physics, not curve-fitting. If an engine delivered constant power P to the wheels from a standstill, then P = force × speed, and integrating gives speed³ = 3Px ÷ m after distance x, with time t = (1.5x)^(2/3) × (m ÷ 2P)^(1/3). Put in 1,320 ft, g = 32.174 ft/s² and 1 hp = 550 ft·lbf/s, and the ideal constants come out near 4.80 for ET and 281 for mph. Fox's 6.269 is about 31% slower than that ideal and his 230 about 18% lower, and the gap is everything a real car loses: a tire cannot transmit full power at zero speed, so the launch is traction-limited; gear changes interrupt the drive; the drivetrain absorbs some power; aerodynamic drag grows with the square of speed; and rolling resistance never lets go. Each constant pair folds those losses into a single number for a population of cars, which is why the race-prepared Hale set sits closer to the ideal. The formulas also take one horsepower number at face value, though a car only makes its peak power at one engine speed; an engine with a broad, flat power band delivers more of that peak across the run than a peaky one rated the same.

Two consequences follow. Because of the cube root, a 1% change in weight or power moves both estimates by only about a third of a percent; 10% less weight makes the car about 3.45% quicker. And within one model, ET × trap speed is the same for every car (1,441.87 for Fox), so the formula always predicts the two moving together. A real slip that breaks that link is telling you something about the launch.

A worked example.

Frequently asked questions.

References& sources.

1. [1]Fox, G. T. (1973). On the Physics of Drag Racing. American Journal of Physics 41(3), 311-313; peer-reviewed analysis behind the named Fox relationship.
2. [2]Omni Calculator. Quarter Mile Calculator; documents and compares Huntington 6.290/224, Fox 6.269/230, and Hale 5.825/234 equations and their empirical scope.
3. [3]SAE International J1349_201804. Engine Power Test Code; authoritative context for standardized as-installed net engine power ratings.
4. [4]FIA. 2026 Drag Racing Technical Regulations; current example of vehicle-, class-, weight-, equipment-, and safety-specific sanctioned rules.
5. [5]NIST. SI Units — Length; exact U.S. customary-to-SI length relationships supporting 1,320 ft = 402.336 m.