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Quarter mile calculator

Enter the race weight and the horsepower. See an estimated elapsed time and trap speed for the quarter mile, or type in a time slip and see the horsepower it suggests.

Car

Race weight

The car as it runs, with the driver and fuel. Enter a weight from {lo} to {hi}.

Engine power

Engine (flywheel) power is the usual input for these formulas. Enter a power figure above zero.

These are Patrick Hale's empirical estimates for a well-sorted car. Real times depend on traction, gearing, the launch, the driver, the track and the weather, so a real time slip can differ by a lot, most often slower.

Horsepower from a time slip

Uses the race weight above. Fill in either box, or both.

Enter an ET from 3 to 40 seconds.

Trap speed

mph

Enter a trap speed above zero.

With more or less power

What the inputs mean

Race weight: the car as it goes down the track, with the driver, fuel and anything else on board. Curb weight alone makes the estimate too quick.
Horsepower: engine power. These formulas are usually used with engine (flywheel) horsepower; wheel horsepower gives a slower estimate.

The formulas

Patrick Hale fitted these to real quarter mile results. They are estimates, not physics from first principles:

ET (seconds) = 5.825 × (weight ÷ hp)^(1/3)trap speed (mph) = 234 × (hp ÷ weight)^(1/3)

Weight is in pounds. The cube root means power pays off less than you might expect: doubling the power cuts the ET by about 21%, not by half.

Turned around, the same formulas give horsepower from a time slip:

hp = weight ÷ (ET ÷ 5.825)^3hp = weight × (trap speed ÷ 234)^3

Worked example

A car with a race weight of 3,500 lb and 400 hp:

3,500 ÷ 400 = 8.75 lb/hp. The cube root of 8.75 is 2.061, so ET = 5.825 × 2.061 = 12.00 seconds. The cube root of 400 ÷ 3,500 is 0.4853, so trap speed = 234 × 0.4853 = 113.6 mph (182.8 km/h).

Going the other way, the same 3,500 lb car running a 12.50 s ET suggests about 354 hp, and a 110 mph trap speed suggests about 364 hp.

Which figure to trust

Trap speed depends mostly on power and weight. ET also depends heavily on the launch: wheelspin, a soft launch or a poor shift all add time but barely change the trap speed. That is why the horsepower worked out from trap speed is usually the steadier of the two. If the two estimates differ a lot, the gap often points to how the car leaves the line. Every number here is an estimate from Hale's formulas, not a prediction for your car.

Questions

What will a 400 hp, 3,500 lb car run in the quarter mile?

Hale's formulas estimate 12.00 seconds at 113.6 mph for a well-sorted car. Traction, gearing, the launch and conditions decide the real time.

How do I calculate horsepower from trap speed?

Horsepower = race weight × (trap speed ÷ 234)^3. A 3,500 lb car trapping 110 mph works out to about 364 hp.

How do I calculate horsepower from quarter mile ET?

Horsepower = race weight ÷ (ET ÷ 5.825)^3. A 3,500 lb car running 12.50 seconds works out to about 354 hp.

Should race weight include the driver?

Yes. Use the weight of the car as it runs down the track, with the driver and fuel. Leaving the driver out makes the estimate too quick.