About the Engine Horsepower Calculator
A drag strip time slip says a lot about a car's power. This calculator uses the two classic quarter-mile formulas to estimate horsepower: one from the trap speed at the finish line and one from the elapsed time. Enter the weight of the car with the driver, choose which figure you have, and set the drivetrain loss to see both the power reaching the wheels and the likely power at the engine's flywheel. It is a quick way to check a tune, compare two cars or see whether a dyno figure is believable.
Updated October 2026
How to use the Engine Horsepower Calculator
- 1Choose trap speed or elapsed time.
- 2Enter the weight of the car including the driver, in pounds or kilograms.
- 3Enter the trap speed in mph or the quarter-mile time in seconds.
- 4Set the drivetrain loss for your type of car.
- 5Read the wheel and flywheel horsepower.
Where the formulas come from
The trap speed and elapsed time formulas were fitted to many real drag strip results decades ago and are still used by racers because they work surprisingly well. The constants 234 and 5.825 make the units come out in horsepower when weight is in pounds and speed in mph.
- Trap speed: hp = W × (mph ÷ 234)³
- Elapsed time: hp = W ÷ (ET ÷ 5.825)³
- Flywheel hp = wheel hp ÷ (1 − loss)
Getting good inputs
Weigh the car with the driver and the fuel you raced with; public weighbridges or scales at the track are ideal. Use the trap speed from the same pass as the time slip. Runs at high altitude or on a hot day produce lower speeds, so compare like with like or correct the figures to standard weather.
Reading the result
If the trap-speed figure is much higher than the elapsed-time figure, the car has power but loses time at the start, often from wheelspin or a slow launch. If both are close, the run was clean. Track changes over a season by watching the trap-speed horsepower; it reacts to tuning changes even when launches vary.
Converting to kW and comparing with brochures
Manufacturers quote engine power at the crankshaft, sometimes in metric horsepower (PS) or kW. The calculator shows kW for the flywheel estimate; 1 hp equals 0.7457 kW and 1 PS is about 0.986 hp. If your estimate is much lower than the brochure figure, check tyre pressures, gearing and the weight you entered before blaming the engine.
Using it at the track
The page loads quickly on a phone, so you can type the figures straight from the time slip in the pits. Nothing is saved or uploaded. An engine figure becomes more useful when you connect it to the rest of the vehicle. Convert it with the Horsepower Calculator, look at turning force with the Torque Calculator, and check speed with the Speed Calculator.
Features
- Trap speed and elapsed time formulas
- Wheel and flywheel horsepower
- Power in kW as well
- Weight in pounds or kilograms
- Adjustable drivetrain loss
- Runs in your browser
Frequently asked questions
How do you calculate horsepower from trap speed?
hp = weight × (mph ÷ 234)³, with the weight in pounds including the driver. A 3,500 lb car trapping 105 mph makes about 316 hp at the wheels.
How do you calculate horsepower from elapsed time?
hp = weight ÷ (ET ÷ 5.825)³. A 3,500 lb car running 13.5 seconds makes about 281 hp.
Why do the two formulas give different answers?
Elapsed time depends heavily on the launch, traction and shifts, while trap speed mostly reflects power. Trap speed is usually the more reliable guide.
Is this wheel or engine horsepower?
The formulas estimate the power that actually moves the car, which is closest to wheel horsepower. The calculator adds your drivetrain loss to estimate flywheel horsepower.
What drivetrain loss should I use?
Roughly 12–15% for rear-wheel-drive manuals, 15–20% for automatics, and 20–25% for all-wheel drive. Front-wheel drive manuals are often around 10–15%.
How accurate is it?
It is an estimate, often within 5–10% for street cars. Aerodynamics, gearing, altitude and weather all affect the result, so use it to compare runs rather than as a dyno replacement.