Which Quarter-Mile Formula Is Most Accurate?
Huntington vs. Fox vs. Hale — the three classic equations tested against real time slips, and which to trust for your car.
Every quarter-mile calculator on the internet runs one of three empirical formulas, all cube-root laws of power-to-weight fitted to real time-slip data decades apart. They agree within a couple of tenths for ordinary cars — the interesting question is where each one drifts, and which output you should believe.
The Three Formulas
| Formula | ET constant | MPH constant | Origin |
|---|---|---|---|
| Huntington (1950s) | 6.290 | 224 | Curve fit of 1950s Hot Rod era time slips |
| Fox (1973) | 6.269 | 230 | Physics derivation, Am. J. Phys. 41, 311 |
| Hale | 6.290 | 234 | Refit for modern tires & aero (NHRA-era data) |
All use ET = C × (Weight ÷ HP)⅓ and MPH = C × (HP ÷ Weight)⅓. Notice ETs barely differ (6.269 vs 6.290 is 0.3%); the real spread is in the MPH constant — 224 vs 234 is a 4.5% difference in predicted trap speed, which cubes into a ~13% difference when you invert the formula to estimate horsepower from a time slip.
Tested Against Real Cars
Using published specs and typical tested results (driver included):
| Car | Tested | Huntington | Fox | Hale |
|---|---|---|---|---|
| MX-5 (181 hp / 2,520 lb) | 14.9 @ 94 | 15.1 @ 93 | 15.1 @ 95 | 15.1 @ 97 |
| Mustang GT (480 / 4,010) | 12.4 @ 112 | 12.8 @ 110 | 12.8 @ 113 | 12.8 @ 115 |
| Corvette Z06 (670 / 3,610) | 10.6 @ 131 | 11.0 @ 128 | 11.0 @ 131 | 11.0 @ 134 |
| Model 3 Perf. (510 / 4,230) | 11.7 @ 115 | 12.8 @ 111 | 12.8 @ 114 | 12.8 @ 115 |
The pattern across all three: ETs run ~0.3–0.4 s conservative for modern cars (launch control and sticky tires beat 1950s-calibrated launches), while trap speeds land within a few MPH — Fox and Hale bracketing the tested value. The EV is the outlier: a full second quicker in ET than any formula predicts (instant torque, AWD launch), yet its trap speed matches Hale almost exactly. Trap speed obeys physics; ET rewards the launch.
So Which Should You Use?
- Predicting your ET/MPH from horsepower: Hale (our quarter-mile calculator default). Expect to run a couple tenths quicker than predicted with a good launch, slower on street tires.
- Estimating horsepower from a time slip: use the trap-speed inversion, HP = W × (MPH ÷ 234)³ — see trap speed HP. ET-based estimates absorb wheelspin and driver error.
- EVs and AWD launch monsters: trust trap-speed math only; every classic formula badly over-predicts their ET.
- Corrections: density altitude shifts everything — a hot, high track adds tenths (see altitude loss).
Frequently Asked Questions
One using Hale's constants, read via trap speed. All the classic formulas predict trap speed within a few MPH for gas cars; ET predictions are always softer because launch quality isn't in the equation.
Launch control, sticky tires and AWD get you through the formula's weakest region — the first 60 feet — faster than the historical data it was fitted on. Underrated factory power figures help too.
Trap speed yes, ET no. EVs launch far harder than the formulas assume, running up to a second quicker in ET while trapping right where power-to-weight predicts.