Engine Horsepower Calculator
Estimate engine horsepower from quarter-mile performance and vehicle weight.
Inputs
HP (Trap Speed method)
273.2HP
HP (ET method)
0.0HP
Average Estimated HP
273.2HP
Vehicle Weight
3,500lbs
Step by step
Trap Speed method: HP = weight × (trap_mph ÷ 234)³
3500 × (100 ÷ 234)³
= 273.2 HP
The 234 constant is derived from aerodynamic terminal-velocity physics.
How it works
These formulas estimate wheel horsepower from a car's measured quarter-mile performance. They were derived by Wallace Racing from large datasets of drag-race results. The trap speed formula is more physics-based (terminal velocity scales with cube root of power); the ET formula is a pure empirical regression. Both assume a stock or mildly modified, reasonably aerodynamic vehicle. Heavily modified or oddly shaped vehicles will show greater error.
Formulas
Trap Speed method
HP = weight (lbs) × (trap speed mph / 234)³
- 234
- Empirical constant
- W
- Vehicle weight in lbs
- v_mph
- Trap speed in mph
Elapsed Time method
HP = weight (lbs) ÷ (ET in seconds / 5.825)³
- W
- Vehicle weight in lbs
- ET
- Quarter-mile elapsed time in seconds
- 5.825
- Empirical constant
Frequently Asked Questions
What weight should I enter?
Use the total vehicle weight at the strip: driver weight + fuel + any passengers. This is heavier than the manufacturer's curb weight. A typical passenger car adds 150–200 lbs over curb weight for the driver and a half-tank of fuel.
Are these formulas accurate for my modified car?
They are accurate to within 3–5% for relatively stock, production-bodied vehicles. Significant aerodynamic modifications (wings, diffusers, drag body kits) alter the relationship between trap speed and power. For highly modified cars, a chassis dyno is more reliable.
My trap speed is 110 mph but the ET method gives a different number than the trap speed method — which is right?
Both methods have ~5% uncertainty. If they disagree significantly, check that you're using the actual vehicle weight (not manufacturer curb weight) and that the run was a clean pass (no wheel spin, good reaction time). The trap speed method is generally more reliable because it is less sensitive to launch technique.
Is this wheel HP or crank HP?
These formulas estimate at-the-wheels (or at-the-rear-axle) horsepower from performance data. Typically crank HP is 10–20% higher than wheel HP depending on drivetrain losses.