Quarter Mile Calculator: ET and Trap Speed
How fast will my car run the quarter mile? Enter the weight of the car with driver and the power at the crankshaft to estimate the elapsed time (ET) and the speed at the finish line. This is a rule-of-thumb estimate, not a timed run: traction, gearing, launch, altitude and weather all change real results.
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Formula
These are published empirical rules of thumb (the Hale ET formula and the Fox trap speed formula), not a physics simulation, so the results are an estimate.
ET (s) = 5.825 x (weight in lb / hp)^(1/3)trap speed (mph) = 234 x (hp / weight in lb)^(1/3)1 kg = 2.20462 lb,1 kW = 1.34102 hp,1 PS = 0.98632 hp,1 mph = 1.609344 km/h
Weight should include the driver and fuel; power is at the crankshaft.
Worked example
A 3000 lb car with 300 hp: weight per hp = 10. The cube root of 10 is 2.1544, so ET = 5.825 x 2.1544 = 12.55 s. The cube root of 0.1 is 0.4642, so the trap speed = 234 x 0.4642 = 108.6 mph (174.8 km/h).
Second example: 2800 lb with 150 hp is 18.67 lb per hp, giving 15.45 s at 88.2 mph.
Use this when
You want a quick sense of straight-line performance from two numbers you already know, or want to see how much a power gain or weight loss would change the time. Power matters less than you might expect: because of the cube root, a 10 percent power gain cuts ET by only about 3 percent.
Typical values
For reference, ordinary family cars typically run roughly 15 to 18 seconds, warm hatchbacks and sports sedans roughly 13 to 15, and high-performance cars under 12. Treat these as typical bands, not guarantees.
Quarter mile vs related calculators
The power to weight ratio calculator gives the ratio this formula is built on. The horsepower calculator finds the power figure if you only know torque and rpm. Neither predicts a time or a trap speed.
Common mistakes
- Using wheel horsepower. The formulas assume crank power, so add roughly 15 percent to a dyno wheel figure.
- Forgetting to add the driver and fuel to the weight.
- Expecting accuracy on a slippery surface or with a poor launch.
- Reading the trap speed as top speed.
Frequently asked questions
How is the quarter-mile time estimated?
With the published Hale formula ET = 5.825 x (weight in lb / hp)^(1/3). A 3000 lb car with 300 hp has 10 lb per hp, the cube root of 10 is 2.154, so ET = 5.825 x 2.154 = 12.55 seconds.
How is the trap speed estimated?
With the Fox formula speed = 234 x (hp / weight in lb)^(1/3) mph. For 300 hp and 3000 lb that is 234 x 0.1^(1/3) = 234 x 0.4642 = 108.6 mph (174.8 km/h).
How accurate is this estimate?
It is a rough guide, often within a few tenths of a second for a well-driven car with good traction. It assumes rated crank power, a sensible launch and sea-level air. Wheelspin, gearing, drivetrain losses and weather can add or remove a second or more.
Should I use crank horsepower or wheel horsepower?
The formulas were fitted to flywheel (crank) horsepower with the driver aboard. If you only know wheel power from a dyno, add roughly 15 percent for drivetrain loss first; for example 255 whp is about 300 hp at the crank.
What is the quarter mile time for a 400 hp car?
A 3500 lb car with 400 hp has 8.75 lb per hp, which gives an estimated ET of 12.0 s and a trap speed of 113.6 mph (182.8 km/h). Real times depend heavily on traction and launch.
How do I use this with kg and kW?
Switch the units and enter the values directly. A 1400 kg car with 150 kW (about 201 hp) gives an estimated ET of 14.47 s and a trap speed of 94.2 mph, or 151.5 km/h.
What is a good quarter mile time?
As typical guides, around 14 to 16 seconds is ordinary for a family car, 13 seconds or less is quick, and under 12 seconds is very fast. The estimate for a 2800 lb, 150 hp car is 15.45 s at 88.2 mph.