Engine Horsepower Calculator
lbs
sec
Estimated Horsepower314.9 HP
You can estimate an engine power without a dyno by working backward from how a car performs at the drag strip. This calculator uses the well-known elapsed-time (ET) method — a form of Hale's equation — to estimate horsepower from a vehicle race weight and its quarter-mile elapsed time. Because heavier cars need more power to run the same ET, weight and time together pin down an approximate flywheel horsepower figure.
Formula
HP = weight ÷ (ET ÷ 5.825)³
- weight
- Vehicle weight in pounds, ideally including driver and fuel
- ET
- Quarter-mile elapsed time in seconds
- 5.825
- Empirical constant in the ET (Hale) horsepower formula
How it works
- Enter the vehicle weight in pounds (ideally race weight, including driver and fuel) and its quarter-mile elapsed time in seconds.
- The calculator divides the ET by the empirical constant 5.825 and cubes the result to get a time factor.
- It then divides the weight by that time factor to estimate horsepower, rounded to two decimal places: HP = weight ÷ (ET ÷ 5.825)³.
Worked example
A 3,500 lb car that runs the quarter mile in 13.0 seconds.
- Time factor = ET ÷ 5.825 = 13.0 ÷ 5.825 = 2.2318.
- Cube it: 2.2318³ ≈ 11.116.
- Horsepower = 3,500 ÷ 11.116 ≈ 314.87.
About 314.87 horsepower.
Frequently asked questions
- What method does this calculator use?
- It uses the quarter-mile elapsed-time (ET) method, a version of Hale's formula that relates a car weight and ET to estimated engine power. It does not use torque and RPM or trap speed — only weight and elapsed time.
- Is the result flywheel or wheel horsepower?
- The ET formula approximates flywheel (crankshaft) horsepower, since it is calibrated against engine output rather than measured at the wheels. Drivetrain losses mean actual wheel horsepower is typically 10-20% lower.
- How accurate is the estimate?
- It is a ballpark figure. Real ET depends on traction, gearing, aerodynamics, launch technique, and conditions, so two cars with the same power can run different times. Use it as an estimate, not a substitute for a dyno.
- What weight should I enter?
- Use the car total race weight at the strip, including the driver, fuel, and any cargo. Because horsepower scales directly with the weight in this formula, leaving out the driver will noticeably understate the estimate.