Quick answer: A 1,600 kg car (3,527 lb with driver) that runs the quarter mile in 13.5 s has about 3,527 ÷ (13.5 ÷ 5.825)³ = 283 hp.
How it's calculated
Drag racers use two empirical rules, published by Roger Huntington and refined by Patrick Hale: from elapsed time, hp = weight ÷ (ET ÷ 5.825)³; from trap speed, hp = weight × (mph ÷ 234)³, with weight in pounds including the driver.
Trap speed depends mostly on power and weight, so it is usually the better estimate. Elapsed time also depends on the launch, so wheelspin or a slow start makes the ET figure read low.
These are rules of thumb fitted to real cars, not exact physics. A dynamometer measures power directly; use this to compare runs or check a claimed figure.
Worked examples
- 1,600 kg (3,527 lb), 13.5 s: 3,527 ÷ (13.5 ÷ 5.825)³ = 283 hp
- 3,527 lb, 105.6 mph (170 km/h): 3,527 × (105.6 ÷ 234)³ = 324 hp
- 3,000 lb, 12.0 s: 3,000 ÷ (12.0 ÷ 5.825)³ = 343 hp
Frequently asked questions
Is this horsepower at the wheels or at the engine?
Neither exactly: it is the power the run implies after traction and drivetrain losses. Treat it as an estimate to compare with dyno or brochure figures.
Which method should I trust?
Trap speed, in most cases, because it is less affected by the launch. A big gap between the two usually means traction was lost.
What weight should I enter?
The race weight: the car with fuel, driver and anything carried during the run.
What is a quarter mile?
402 metres (1,320 ft), the classic drag-racing distance. Your ET and trap speed are both taken over it.
How much power is lost through the drivetrain?
Usually somewhere around 10 to 20% between the engine and the wheels, depending on the gearbox and whether it's front-, rear- or all-wheel drive. Only compare numbers measured the same way.
Does the weather change my times?
It does. Hot, humid air has less oxygen in it, so the engine makes less power. The same car can run noticeably slower on a sticky summer evening at Dubai Autodrome than on a cool winter night.