Quarter Mile Calculator

Quarter Mile Calculator estimates a car’s ET and trap speed from its weight and horsepower, or its horsepower from a timeslip. It also gives the eighth-mile ET and power to weight.

Quarter Mile Calculator for ET and Trap Speed

A quarter mile time is how long a car takes to cover 1,320 ft from a standing start. Drag racers call it the elapsed time, or ET. The speed crossing the finish line is the trap speed. The Quarter Mile Calculator estimates both from two numbers, the weight of the car and its engine power.

Calculate has three settings. ET and Trap Speed is the usual direction. Horsepower From ET and Horsepower From Trap Speed work backward from a timeslip to the power the engine must be making. Weight should include the driver and fuel, since that is what the engine has to move.

The default is a 3,400 lb car with 380 hp at the flywheel. It comes out at 12.09 s and 112.72 mph. The cards add an eighth-mile time of 7.75 s, an average acceleration of 0.42 g, a weight-to-power ratio of 8.95 lb per hp, and the answers from two other formulas.

Quarter Mile Formulas

Every quarter mile formula has the same shape. Elapsed time rises with the cube root of weight divided by power, and trap speed rises with the cube root of power divided by weight. Only the constants differ. With weight W in pounds and power P in horsepower, the Hale version is below.

$$ET = 5.825 \times \sqrt[3]{\frac{W}{P}}$$

$$MPH = 234 \times \sqrt[3]{\frac{P}{W}}$$

The Quarter Mile Calculator carries three published sets of constants. Roger Huntington fitted the first to magazine test data in the 1950s.

Geoffrey Fox, a physics professor, gave the cube-root law a theoretical basis in the American Journal of Physics in 1973. Patrick Hale’s constants are the most recent of the three, and the tool opens with them.

The gap between them answers the question of which is the most accurate 1/4 mile calculator. None is exact. The three disagree by almost a second on ET here, but by less than 5 mph on trap speed. Trap speed depends mainly on power and weight, while ET also depends on the launch.

The cube root has a practical meaning. Doubling the power does not halve the time. It cuts the ET by about 21% and raises the trap speed by 26%. On the default car, 10% more power is worth 0.38 s, and removing 100 lb is worth 0.12 s.

ET by Weight-to-Power Ratio

Since only the ratio matters, one table covers every car. Divide the race weight by the flywheel horsepower and read across. These are Hale figures, the same ones the Quarter Mile Calculator shows when it opens.

Whether a time is good depends on the car. A 15 second run is ordinary for a family car. An 11 second quarter mile is fast for anything street legal, and it takes about 6.7 lb per hp. A 9 second run needs under 3.7 lb per hp, which is race-car territory.

1/4 Mile HP Calculator

The two horsepower settings of the Quarter Mile Calculator turn a timeslip into an engine power figure. For a target time, they show what a build needs. The table gives the flywheel power a 3,400 lb car needs for each ET by the Hale formula.

So a quarter mile in 10 seconds means an average of 90 mph from a standstill, and a finish-line speed near 136 mph. Each second gets harder to find. Going from 12 s to 11 s takes 116 more horsepower, and going from 10 s to 9 s takes 250.

Working from a real slip, a 3,400 lb car that ran 12.5 s works out to 344.1 hp, and one that trapped at 110 mph works out to 353.2 hp. When the two disagree, trust the trap speed. A poor launch adds time without taking much speed off the top end.

Wheel horsepower and AWD cars

The formulas expect flywheel power. A dyno reads power at the wheels, which is lower by the drivetrain loss. Set Power Is Measured At to The Wheels and the Quarter Mile Calculator converts first. With a 15% loss, 323 hp at the wheels is 380 hp at the flywheel.

There is no separate 1/4 mile calculator for AWD, because the formula has no input for which wheels are driven. An all-wheel-drive car usually loses a little more power in the drivetrain, so a higher loss figure suits it. Its better grip off the line shows up in the ET and hardly at all in the trap speed. Grip itself is the subject of the Friction Calculator.

1/4 to 1/8 Mile ET

Many tracks run the eighth mile, which is 660 ft. A car is still accelerating at that point, so the eighth takes much more than half the time. The usual conversion divides the quarter mile ET by 1.56, and the second card of the Quarter Mile Calculator does this for every result.

To go the other way, multiply an eighth-mile time by 1.56. A 7.75 s eighth points to a 12.09 s quarter. The default car covers the back half at an average of 103.66 mph. The average acceleration over the whole run can be checked in the Magnitude of Acceleration Calculator.

The quickest drag racing classes no longer run the full distance. In 2008 the NHRA shortened Top Fuel and Funny Car races to 1,000 ft from the traditional 1,320 ft, as SEMA News reported at the time. Their times are not quarter mile times.

How Far Is a Quarter Mile?

A quarter mile is 1,320 feet, or 440 yards. In metric it is 402.336 meters, which is 0.402 km. It is usually written 1/4 mile. One lap of a standard 400 m running track is 2.34 m short of it, which is the easiest way to measure one on foot.

At a walking pace of 3 mph, a quarter mile takes 5 minutes. With a 2.5 ft stride it is about 528 steps, and 10,000 steps of that length come to 4.73 miles. A 2 minute quarter mile is 7.5 mph, a steady running pace. Walking a full mile in 10 minutes would mean 6 mph, which is a jog for most people.

A mile and a kilometer are not the same. One mile is 1.609 km, so a quarter mile is longer than a quarter kilometer. A quarter kilometer, 250 m, is 0.155 miles. Result Units in the Quarter Mile Calculator switches speeds to km/h and power to kW.