How to use the quarter mile time estimator
- Enter the weight of the vehicle with the driver, in pounds, and the horsepower.
- Read the estimated elapsed time and the estimated speed at the finish line.
- Open the advanced options to change the two constants. They are example values from published rules of thumb, and several different sets circulate.
- Use the table to see how the estimate moves if your real power is a little higher or lower than the figure you entered.
Formula
Elapsed time = c x (weight / horsepower)^(1/3)
Finish speed = t x (horsepower / weight)^(1/3)
Default constants: c = 6.290 and t = 224
This is an empirical rule of thumb. Different published versions use different constants, such as 6.269 or 5.825 for the time. This page does not claim to know which set is correct for your car.
A 3,000 lb car with 300 hp
Weight divided by horsepower is 3,000 / 300 = 10. The cube root of 10 is 2.1544. Elapsed time = 6.290 x 2.1544 = 13.55 seconds.
Finish speed = 224 x (300 / 3,000)^(1/3) = 224 x 0.4642 = 103.97 mph, shown as 104 mph. With the time constant changed to 5.825 the same car comes out at 12.55 seconds, which shows how much the choice of constant matters.
How the estimate works
The formula says that elapsed time grows with the cube root of the weight to power ratio. Doubling the weight to power ratio does not double the time, because the cube root grows slowly. It was fitted to measurements of cars at the drag strip, so it describes a rough relationship between those quantities but it is not a physical law.
Names are often attached to the constants, and different sources attach different names to the same numbers. Because that attribution is inconsistent, this page shows the constants themselves and lets you change them.
Why real results differ
Traction, launch technique, gearing, tire grip, altitude and air temperature, the track surface and the driver all change a real run, often by more than a second. Cars with different drivelines can launch very differently, and a heavy turbocharged car may be quicker or slower than the formula suggests.
The horsepower should be the figure at the crank unless your constants were fitted to wheel power. Treat the estimate as a rough guide. To compare vehicles by weight and power alone, see the power to weight ratio calculator, and for the horsepower itself the horsepower from torque calculator.
Assumptions and limits
- The constants are example values, and you can change them.
- Weight includes the driver and any fuel and cargo you want counted.
- Horsepower is the engine figure at the crank.
- The estimate assumes a good launch, a flat track and normal air, and ignores the driver and the vehicle's traction.
Frequently asked questions
How do you estimate quarter mile time from weight and horsepower?
Divide weight in pounds by horsepower, take the cube root, and multiply by a constant such as 6.290. For 3,000 lb and 300 hp that gives about 13.55 seconds.
How accurate is the estimate?
It is a rough guide. Traction, gearing, launch and conditions can move a real time by a second or more, so use the result to compare similar cars rather than to predict a run.
Why are there different constants?
Several empirical versions of the formula are published, fitted to different sets of cars, and they give different answers. The constants on this form can be edited so you can use the set you prefer.
Should I use crank or wheel horsepower?
This page does not know which kind of horsepower a set of constants was fitted to. A wheel horsepower figure is lower than a crank figure for the same car, so it gives a slower estimate than a crank figure would.