Engine Horsepower Calculator
Estimate engine horsepower from quarter-mile performance and vehicle weight.
About Engine Horsepower Calculator
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
Worked example
With the default values below, Engine Horsepower Calculator returns HP (Trap Speed method) of 273.2. Change any input above and every figure here is recalculated in your browser.
Inputs used
- Methodone of Trap Speed method (more accurate), Elapsed Time (ET) method, Both methods
- Trap Speed method (more accurate)
- Vehicle Weight (with driver & fuel)accepts 500–20,000 lbs
- 3,500 lbs
- Trap Speedaccepts 30–300 mph
- 100 mph
- Elapsed Time (ET)accepts 5–30 sec
- 13.5 sec
Results
- HP (Trap Speed method)
- 273.2 HP
- HP (ET method)
- 0.0 HP
- Average Estimated HP
- 273.2 HP
- Vehicle Weight
- 3,500 lbs
Step by step
- 1Trap Speed method: HP = weight × (trap_mph ÷ 234)³= 273.2 HPThe 234 constant is derived from aerodynamic terminal-velocity physics.
Assumptions and limits
- The result is derived only from the 4 inputs listed above. Anything not among them is not part of the calculation.
- In particular, this calculator does not take inflation, tax, fees or charges or compounding frequency into account.
- Figures are rounded for display to 1 decimal place, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.
How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.
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.