How to use the 1/4 Mile Calculator
- Enter the vehicle weight, including driver and fuel.
- Enter horsepower, noting whether it is at the crank or the wheels.
- Read the estimated elapsed time and trap speed.
- Treat the result as a well-prepared best case, not a street figure.
How the calculation works
The standard approximations are ET ≈ 5.825 × (weight ÷ HP)^(1/3) and trap speed ≈ 234 × (HP ÷ weight)^(1/3). Both depend on the power-to-weight ratio through a cube root, which means a 10% weight reduction and a 10% power increase produce almost identical gains — and weight reduction also improves braking, cornering and tyre life at the same time.
These formulas assume competent traction and a prepared surface. Real results depend heavily on launch: a rear-drive car spinning its tyres off the line loses several tenths that no power figure recovers, while drivetrain type, gearing, aerodynamics above about 100 mph, and air density all shift the outcome. Trap speed is the more robust indicator of engine output because it is less sensitive to launch quality than elapsed time.
ET ≈ 5.825 × (weight / HP)^(1/3) ; trap mph ≈ 234 × (HP / weight)^(1/3)Source: Roger Huntington's empirical quarter-mile approximations, long-standing drag racing estimation practice.
Worked example
A 3,400 lb car with 395 hp at the crank, then the same car 250 lb lighter.
- weight/HP = 3,400 / 395 = 8.608; cube root = 2.051; ET ≈ 5.825 × 2.051 = 11.95 s.
- HP/weight = 0.11618; cube root = 0.4879; trap ≈ 234 × 0.4879 = 114.2 mph.
- At 3,150 lb: ET ≈ 11.65 s, trap ≈ 117.2 mph.
About 11.95 s at 114 mph, improving to roughly 11.65 s at 117 mph with 250 lb removed — a gain that would otherwise need around 30 extra horsepower.
Frequently asked questions
How accurate is the estimate?+
Within a few tenths for a well-prepared car with good traction. Street tyres, a poor launch or a cold surface can easily cost half a second or more.
Should I use crank or wheel horsepower?+
The formulas were derived around crank figures. Using wheel horsepower will predict a slower time than the car actually runs, so be consistent about which you use when comparing.
Why does losing weight help as much as adding power?+
Both act on the power-to-weight ratio through a cube root, so proportional changes give similar gains — and weight reduction improves braking and handling as well.
Is trap speed or ET the better measure of engine output?+
Trap speed, because it depends less on launch quality. Two cars with identical power can differ substantially in ET while trapping within a mile per hour of each other.
Last reviewed August 31, 2026. We review this page whenever the underlying formula, tax year, published rate or standard changes.