Horsepower from Trap Speed Calculator
This calculator estimates engine horsepower from a quarter-mile trap speed using the well-known drag racing formula HP = W * (mph / 234)^3, where W is the total vehicle weight in pounds and mph is the speed through the timing lights at the end of the quarter mile. This empirical formula has been widely used in motorsport for decades to cross-check dynamometer readings against real-world track performance. It is especially useful when a dyno is unavailable but you have a confirmed trap speed from a sanctioned drag strip. The constant 234 was determined from extensive track data correlating terminal velocity to measured power output.
Trap speed horsepower formula
HP = Weight (lb) * (Trap Speed (mph) / 234)^3
Flywheel HP (RWD estimate) = HP / 0.85. The drivetrain efficiency factor of 0.85 is typical for a rear-wheel drive car with an automatic or manual transmission. Use 0.80 for AWD. These factors are estimates; actual loss varies by transmission type and condition.
Tips for accurate results
- Use a sanctioned drag strip with NHRA-certified timing equipment for the most accurate trap speed reading.
- Weigh the vehicle at a scale with the driver and fuel level as run. Corner-weight scales give the most accurate total weight.
- Run multiple passes and average the trap speeds. Environmental conditions (temperature, altitude, humidity) affect results.
- The formula assumes moderate aerodynamic drag. Purpose-built streamliner cars or high-downforce vehicles may deviate from the estimate.
Frequently asked questions
What is the trap speed horsepower formula?
The formula HP = W * (mph / 234)^3 relates a vehicle's quarter-mile trap speed (mph) and total weight (lb) to estimated rear-wheel horsepower. It was developed empirically from drag racing data and gives a close approximation when drivetrain losses are moderate.
What weight do I use?
Use the total vehicle weight including driver, passengers, and fuel at the time of the run. Removing weight is one of the most effective ways to improve performance: a 100 lb reduction at 400 hp is worth roughly the same ET improvement as adding significant horsepower.
Is this flywheel or wheel horsepower?
The trap-speed formula estimates wheel horsepower (WHP), which is the power actually delivered to the ground. To estimate engine (flywheel) horsepower, divide by the drivetrain efficiency: typically 0.85 for rear-wheel drive or 0.80 for all-wheel drive.
How accurate is this formula?
The formula is accurate within about 5-10% for vehicles with moderate aerodynamic drag and typical drivetrain losses. High-downforce cars and vehicles with extreme tire grip may show slightly different results because the formula does not model aerodynamic drag coefficient.
What is a typical quarter-mile trap speed for a stock vehicle?
A typical economy sedan traps at 80-90 mph; a sporty muscle car at 100-115 mph; a dedicated drag car at 130-150+ mph. NHRA safety rules require roll cages above 135 mph.
Official sources
- SAE International: sae.org - vehicle performance and powertrain standards.
- NHTSA: nhtsa.gov - vehicle weight and safety data.
Reviewed by the CalculatorHub team, edited by James Graham, 15 June 2026. See our methodology.