Horsepower Calculator (Track)

The Formula

The Hale (Patrick Hale) formula estimates engine horsepower from quarter-mile drag strip performance. Using both ET and trap speed gives the most accurate estimate by averaging the two independent calculations.

Variable Definitions

How to Use This Calculator

1. 1

   Select your input mode: ET only, trap speed only, or both (most accurate).

2. 2

   Enter vehicle weight including driver and fuel.

3. 3

   Enter the quarter-mile ET and/or trap speed from your timeslip.

4. 4

   Select drivetrain type to estimate wheel horsepower.

5. 5

   Review estimated crank and wheel horsepower, power-to-weight ratio, and performance tier.

Horsepower estimated from quarter-mile ET and trap speed using the Hale formula

Understanding the Concept

The Hale formula correlates drag strip performance with engine power. ET-based calculation uses the time to complete 1,320 feet; speed-based uses the vehicle velocity at the finish line. The 5.825 constant in the ET formula represents the cubic root relationship between time and power, while 234 in the speed formula accounts for aerodynamic and rolling resistance at high speed. Averaging both methods cancels out individual errors: ET can be affected by traction off the line, while speed is affected by aerodynamic drag and gearing. Practical example: a 3,500 lb car runs a 13.0-second quarter-mile at 108 mph. Using both modes: HP from ET = 3,500 / (13.0 / 5.825)³ = 3,500 / (2.232)³ = 3,500 / 11.12 = 314.7 HP. HP from speed = 3,500 × (108 / 234)³ = 3,500 × (0.462)³ = 3,500 × 0.0984 = 344.4 HP. Average = (314.7 + 344.4) / 2 = 329.6 HP at the crank. With an RWD drivetrain (18% loss), wheel HP = 329.6 × 0.82 = 270.3 WHP. Power-to-weight = 3,500 / 329.6 = 10.6 lbs/HP (Performance category). Edge cases: for high-horsepower cars running on drag radials with excellent traction, the ET method can overestimate power because the car hooks up better than the formula assumes — the Hale formula was calibrated for street tires. For AWD cars, the ET method is more accurate than speed because AWD launches are more consistent, but the speed method may under-read because AWD drivetrain losses are higher (25%). For very high altitudes (e.g., Denver at 5,280 ft), air density is about 17% lower than sea level, which reduces naturally aspirated engine power by about 3-4% per 1,000 ft. The Hale formula does not correct for altitude. For turbocharged cars, altitude has minimal effect on power since the turbo compresses air to a set boost level regardless of ambient density. For electric vehicles, the Hale formula does not apply because EVs have a fundamentally different power delivery (constant torque, no shifting, no clutch dump launches). Hybrid powertrains with instant torque fill also deviate from the formula assumptions.

Frequently Asked Questions

Sources & References