Horsepower from Trap Speed Calculator
Trap speed, the speed a car reaches as it crosses the quarter-mile finish line, is one of the most reliable real-world indicators of horsepower. Because it reflects how much power was needed to move the car's mass over the distance, it captures engine output better than elapsed time alone. This calculator estimates horsepower from trap speed and vehicle weight using the widely cited Hale formula: horsepower equals weight multiplied by the cube of trap speed divided by 234. The constant 234 is an empirical figure that bundles together typical aerodynamic drag and drivetrain losses across many measured runs. You enter the trap speed in miles per hour and the vehicle's race weight in pounds, the weight as it actually ran including driver and fuel, and the tool returns an estimate of flywheel horsepower. Every figure is computed deterministically from the formula shown below, never estimated by guesswork, so the worked example reconciles exactly with the result on screen. Trap-speed estimates are usually within a few percent of dynamometer numbers for a clean pass. Remember that the result is crank horsepower, somewhat higher than the wheel horsepower a chassis dyno would read. Use this tool to gauge an engine's output from a track slip or to compare builds.
The Hale trap-speed formula is HP = weight x (trap speed / 234)^3. For a trap speed of 120 mph and a race weight of 3,500 lb, the estimated horsepower is 472.02 hp.
Trap-speed horsepower formula
HP = weight x ( trap speed / 234 )^3
trap speed = quarter-mile finish speed in mph
weight = race weight in pounds (with driver)
234 = empirical constant (drag and drivetrain losses)
The cube of trap speed divided by 234 reflects how power scales with speed, and multiplying by weight accounts for the mass that had to be accelerated. The result estimates flywheel horsepower for a typical quarter-mile pass.
Worked example
A car traps 120 mph at a race weight of 3,500 lb.
- trap speed / 234 = 120 / 234 = 0.512821
- (0.512821) cubed = 0.134864
- HP = 3,500 x 0.134864 = 472.02 hp
The estimated flywheel horsepower is about 472.02. These are the calculator's default inputs, so the result above matches the widget exactly.
Trap-speed horsepower: frequently asked questions
What is trap speed?
Trap speed is the speed a vehicle reaches as it crosses the finish line of a standing quarter-mile run, measured in miles per hour. Because it reflects how much power moved the car's mass over the distance, trap speed is a reliable indicator of horsepower, often more so than elapsed time alone.
What formula does this use?
It uses the widely cited Hale trap-speed formula: horsepower equals weight times the cube of trap speed divided by 234, all to the appropriate powers, written as power equals weight times (trap speed divided by 234) cubed. The constant 234 captures aerodynamic and drivetrain effects averaged across many real runs.
Should I use race weight or curb weight?
Use the vehicle's weight as it ran, including the driver and any fuel, because that is the mass the engine actually accelerated. This race weight is usually a couple of hundred pounds above the listed curb weight once the driver is added. Enter the figure that matches the run.
How accurate is the trap-speed estimate?
Trap-speed estimates are typically within a few percent of dynamometer figures for a well-run pass, which is why they are popular. Accuracy drops if the launch was poor, traction was lost, or weather and altitude were unusual, since the constant assumes broadly average conditions.
Is this flywheel or wheel horsepower?
The Hale formula estimates flywheel horsepower, the engine's crank output, because it is calibrated against manufacturer power figures. Wheel horsepower measured on a chassis dynamometer is lower due to drivetrain losses, usually by roughly 10 to 15 percent.
Official sources
- Vehicle performance and safety reference: US National Highway Traffic Safety Administration (NHTSA). As at 25 June 2026.
Reviewed by the CalculatorHub team, edited by James Graham, 25 June 2026. See our methodology. This is general information, not financial, tax, legal or investment advice.