Engine Horsepower Calculator
Estimate engine horsepower from vehicle weight and quarter-mile elapsed time or trap speed using the classic drag racing performance formulas.
How It's Calculated
Formula
HP_{ET} = \dfrac{\text{weight}}{\left(\dfrac{ET}{5.825}\right)^3} \qquad HP_{MPH} = \text{weight} \times \left(\dfrac{\text{trap speed}}{234}\right)^3This calculator estimates engine horsepower from a vehicle's weight and its quarter-mile performance, using the Hale-style pair of classic, internally-consistent empirical drag racing formulas. The ET (elapsed time) method uses ET = 5.825 x (weight / HP)^(1/3), solved for HP. The trap speed method uses MPH = 234 x (HP / weight)^(1/3), solved for HP. Both formulas are long-published, widely-used curve fits of real-world quarter-mile performance data — they are estimates, not precise measurements, and they implicitly bake in typical real-world losses (drivetrain friction, aerodynamic drag, tire slip) for a representative vehicle. This calculator reports both methods' results as estimated horsepower; it does not claim to isolate a precise wheel-horsepower or crank-horsepower figure, since the underlying formulas do not support that distinction with certainty.
Worked Examples
3200 lb vehicle, 12.82 s ET, 106.32 mph trap speed
- ET method: HP = 3200 / (12.82 / 5.825)^3 = approximately 300 HP.
- Trap speed method: HP = 3200 x (106.32 / 234)^3 = approximately 300 HP.
Frequently Asked Questions
Is this a measured horsepower figure?
No. This is an estimated horsepower figure derived from empirical formulas that curve-fit typical quarter-mile performance data. It is not a dyno measurement and does not account for a specific vehicle's individual gearing, tire grip, aerodynamics, or driver skill.
Is this the same as the Horsepower Calculator?
No. The Horsepower Calculator computes horsepower directly from engine torque and RPM using an exact physical definition (HP = torque x RPM / 5252). This calculator instead estimates horsepower indirectly from vehicle weight and quarter-mile elapsed time or trap speed, using empirical curve-fit formulas — a different method for a different situation.
Why do the ET method and trap speed method give slightly different results?
Both are independent empirical approximations built from different measured quantities (elapsed time vs. trap speed). Real-world runs rarely fit either curve exactly, so the two methods can diverge somewhat; neither is authoritative over the other.
Does this tell me wheel horsepower or crank (flywheel) horsepower?
Neither precisely. The formulas' constants were fit to representative real-world runs that already include drivetrain and aerodynamic losses, so the result sits somewhere between the two without a verified way to separate them further.