A drag strip measures two numbers for every run over the quarter mile: the elapsed time (ET) and the speed through the last 66 feet (the trap speed). Long before anyone reaches the track, both can be estimated from just two figures — how much the car weighs and how much power the engine makes. This calculator applies two well-known empirical drag-strip formulas, Hale and Fox, to give you a realistic target.
How to use the quarter mile calculator
- Enter the weight of the car with the driver, in pounds or kilograms.
- Enter engine power at the clutch, in horsepower or kilowatts.
- Choose a formula. Hale is the default; Fox gives slightly slower, more conservative numbers.
- Read the ET, the trap speed in mph and km/h, and the weight-per-horsepower ratio.
If you only know torque and engine speed, convert first with the torque to horsepower calculator or the horsepower to torque calculator.
Quarter mile formulas
Fox: ET = 6.269 × (W ÷ P)^(1/3), trap speed = 230 × (P ÷ W)^(1/3)
- W — total weight in pounds, including driver
- P — peak engine power in horsepower at the clutch
- The cube root appears because power is spent against air drag and inertia; the constants are fitted to measured runs.
Worked example: 3,500 lb, 400 hp
- Weight per horsepower: 3,500 ÷ 400 = 8.75 lb/hp.
- Cube root: 8.75^(1/3) = 2.062.
- Hale ET: 5.825 × 2.062 = 12.00 s.
- Hale trap speed: 234 ÷ 2.062 = 114 mph.
- Fox gives 12.92 s at 112 mph for the same car.
Typical quarter-mile estimates
Every row was calculated with the two formulas above.
| Weight | Power | lb per hp | Hale ET @ trap speed | Fox ET @ trap speed |
|---|---|---|---|---|
| 2,800 lb | 200 hp | 14.0 | 14.04 s @ 97 mph | 15.11 s @ 95 mph |
| 3,000 lb | 300 hp | 10.0 | 12.55 s @ 109 mph | 13.51 s @ 107 mph |
| 3,500 lb | 400 hp | 8.8 | 12.00 s @ 114 mph | 12.92 s @ 112 mph |
| 3,800 lb | 500 hp | 7.6 | 11.45 s @ 119 mph | 12.33 s @ 117 mph |
| 3,400 lb | 650 hp | 5.2 | 10.11 s @ 135 mph | 10.88 s @ 132 mph |
| 3,200 lb | 800 hp | 4.0 | 9.25 s @ 147 mph | 9.95 s @ 145 mph |
How to read the result
The estimate describes a car that launches cleanly and is geared to stay in the powerband. Trap speed is the more trustworthy number, because it is set mostly by power and weight; ET also depends on the launch, tires and gearing, and shaves or adds tenths from run to run. If your real trap speed is close to the estimate but your ET is slow, the problem is traction or the launch, not power.
Power-to-weight is the most useful thing to improve. Because both formulas use a cube root, each 10% added power gains roughly 3% in trap speed and 3% off the ET; each 10% of weight removed gains the same. Gearing can matter too: the engine RPM calculator and gear ratio calculator show where the engine will be at the finish line, so you can check that you cross the line near peak power rather than past the rev limiter.
Conditions move the numbers. Hot air, high altitude and humidity all reduce the power an engine makes, so the same car runs slower on a summer afternoon in the mountains than on a cool evening at sea level. Use the calculator for setting goals and comparing modifications, not for guaranteed times.
Sources
The Hale and Fox constants and the worked 975 hp, 1,788 lb example (Hale: 7.13 s at 191.17 mph; Fox: 7.673 s at 187.9 mph) are from the Omni Calculator quarter-mile page, which cites the Fox (1973) and Hale (1980s) empirical fits. Table values were calculated from those constants.
Frequently asked questions
What is ET and trap speed?
ET (elapsed time) is how long the car takes to cover the 1,320 feet of a quarter mile from a standing start. Trap speed is the speed measured in the last stretch before the finish line, and it reflects engine power more directly than ET does.
Which power figure should I enter?
Enter peak net power at the clutch (flywheel horsepower). Wheel horsepower from a dyno reads lower because of drivetrain losses; if you only have wheel power, estimate a few extra percent before entering it.
What is the difference between the Hale and Fox formulas?
Both fit drag-strip data with a cube-root relationship. Hale uses 5.825 and 234; Fox (1973) uses 6.269 and 230 and gives slightly slower, more conservative times. Choose either — they differ by under 10% in ET.
Why is my real time slower than the estimate?
The formulas assume good traction, correct gearing and a well-launched run. Tires that spin, a heavy launch limit, high altitude or hot air, a poor shift or an automatic with a slow converter all add time.
Does weight include the driver?
Yes. Use the total race weight: the curb weight plus the driver, any passengers and cargo, and about the fuel you will carry.