Quarter-Mile ET Calculator

The formulas

ET = 5.825 × (weight ÷ power)^1/3
Trap speed = 234 × (power ÷ weight)^1/3

Weight is in pounds and power in horsepower. The cube root is doing the real work: doubling power does not halve the time, it cuts it by about 21%, because the ratio only falls to one half and its cube root is 0.794.

Worked examples

250 hp, 3,307 lb → 13.23 lb/hp → 13.78 s at 98.9 mph
500 hp, 3,307 lb → 6.61 lb/hp → 10.93 s at 124.7 mph
200 hp, 2,800 lb → 14.00 lb/hp → 14.04 s at 97.1 mph
400 hp, 4,000 lb → 10.00 lb/hp → 12.55 s at 108.6 mph
250 hp, 2,400 lb → 9.60 lb/hp → 12.47 s at 106.3 mph

The second example is the useful one: doubling power from 250 to 500 buys 2.85 seconds, not 6.9. The last two show the same lesson from the weight side — 400 hp in a 4,000 lb car is slower than 250 hp in a 2,400 lb one, despite 150 more horsepower.

Why the estimate drifts

This is a rule of thumb fitted to street cars, not a simulation. It assumes a hooked-up launch, decent tyres and sea-level air. Wheelspin, a tall first gear, a slipping clutch or a heat-soaked engine all add time, and a well-prepared car on drag radials can beat the number by half a second or more.

Altitude and temperature matter through air density: roughly 3% power per 1,000 feet of density altitude, which moves ET by a couple of tenths on a hot day. Trap speed is less sensitive to traction than ET, so it is the better indicator of real power.

EVs distort the pattern because torque arrives instantly and there is no gearshift, so they often beat the estimate while trapping lower than a comparable petrol car.

Using it well

Enter flywheel horsepower, not wheel horsepower, if the figure came from a manufacturer; a chassis dyno reading is already at the wheels and will be 10–15% lower. Use race weight with the driver aboard, not the kerb weight from the brochure.

Frequently Asked Questions