Calculate horsepower from quarter or eighth mile trap speed and race weight, or solve for the trap speed and weight a given horsepower supports.
Horsepower from Trap Speed Formula
HP = W * (V / 234)^3
- HP = estimated engine (flywheel) horsepower
- W = vehicle weight as raced, in pounds, including driver and fuel
- V = quarter-mile trap speed in miles per hour
- 234 = Hale’s empirical constant, fit from drag strip data
The same relationship can be rearranged to solve for the other two variables, which is how the trap speed and race weight modes of the calculator work:
V = 234 * (HP / W)^(1/3)
W = HP * (234 / V)^3
The horsepower mode takes your race weight and trap speed and returns estimated flywheel horsepower. The trap speed mode predicts the finish-line speed a given horsepower and weight should produce. The race weight mode tells you the heaviest a car can be and still trap a given speed with a given horsepower, which is useful when deciding how much weight to pull out of a car.
The formula method setting swaps the constant 234 (Hale) for 230 (Fox) or 224 (Huntington), or averages all three. If you run an eighth-mile track, the calculator multiplies your eighth-mile trap speed by 1.25 to get a quarter-mile equivalent before applying the formula. The optional drivetrain loss field converts the flywheel estimate to approximate wheel horsepower.
Estimated Horsepower by Trap Speed and Race Weight
The table below uses Hale’s formula (constant 234) with quarter-mile trap speeds. Find your trap speed row and the column closest to your as-raced weight to sanity-check the calculator or a friend’s time slip.
| Trap Speed (mph) | 2,500 lb | 3,000 lb | 3,500 lb | 4,000 lb |
|---|---|---|---|---|
| 90 | 142 hp | 171 hp | 199 hp | 228 hp |
| 100 | 195 hp | 234 hp | 273 hp | 312 hp |
| 110 | 260 hp | 312 hp | 364 hp | 416 hp |
| 120 | 337 hp | 405 hp | 472 hp | 539 hp |
| 130 | 429 hp | 514 hp | 600 hp | 686 hp |
| 140 | 535 hp | 642 hp | 750 hp | 857 hp |
The three empirical constants come from different data sets, and the one you pick changes the answer more than most people expect. For a 3,500 lb car trapping 110 mph, the spread looks like this:
| Method | Constant | Estimate at 3,500 lb, 110 mph | Best suited for |
|---|---|---|---|
| Hale | 234 | 364 hp | Well-tuned drag cars with good traction |
| Fox | 230 | 383 hp | Balanced estimate for typical street/strip cars |
| Huntington | 224 | 414 hp | Street cars and older vehicles with more drag and loss |
One practical note that separates trap speed from elapsed time: trap speed is nearly insensitive to launch quality. A bad 60-foot time can add several tenths to your ET while barely lowering your finish-line speed, so trap speed is the more reliable of the two numbers for estimating horsepower. If your ET looks slow but your trap speed is healthy, the power is there and the launch is the problem.
Example Problems
Example 1: A car weighs 3,200 lb as raced and traps 115 mph in the quarter mile. HP = 3200 * (115 / 234)^3 = 3200 * 0.1187 = 380 hp at the flywheel. That works out to about 8.4 lb per horsepower.
Example 2: A truck weighs 3,600 lb and traps 88 mph on an eighth-mile track. Convert to a quarter-mile equivalent first: 88 * 1.25 = 110 mph. Then HP = 3600 * (110 / 234)^3 = 3600 * 0.1039 = 374 hp.
Frequently Asked Questions
Does this estimate wheel horsepower or crank horsepower?
The constants were fit to as-installed engine output, so the result approximates flywheel (crank) horsepower for a typical drivetrain. To approximate wheel horsepower, enter a drivetrain loss percentage in the advanced options, commonly 12 to 15 percent for rear-wheel drive and 15 to 20 percent for all-wheel drive.
Why use trap speed instead of elapsed time to estimate horsepower?
Elapsed time depends heavily on traction, launch technique, and reaction to track prep, while trap speed mostly reflects power-to-weight over the full pass. Two identical cars can differ by half a second in ET from a bad launch but trap within 1 mph of each other. That makes the trap speed method the more repeatable horsepower estimate.
What weight should I enter?
Use the weight as it crosses the finish line: vehicle, driver, and remaining fuel. The formula is linear in weight, so a 100 lb error on a 3,500 lb car shifts the estimate by about 3 percent, roughly 10 hp on a 360 hp result. If you have never scaled the car, add 150 to 200 lb to the published curb weight for the driver and gear.
