Quarter-Mile ET & Trap Speed Estimator
Estimate quarter-mile elapsed time and trap speed from vehicle weight and horsepower. Includes 1/8 mile estimates.
Total vehicle weight with driver and fuel
Wheel horsepower for most accurate results
How These Estimates Work
This calculator uses the Brock/Huntington formula, which has been the standard drag racing estimation method for decades. It calculates ET and trap speed purely from the power-to-weight ratio.
ET = 5.825 × (Weight / HP)^(1/3)
Trap Speed = 234 × (HP / Weight)^(1/3)
Accuracy and Limitations
These formulas are typically within 0.5–1.0 seconds of real-world results for a well-hooked street car with a decent launch. However, they don't account for:
- Traction: The single biggest variable. A car that spins the tires will be much slower than predicted.
- Launch technique: A good 60-foot time can make or break an ET.
- Gearing: Poorly matched gear ratios can leave power on the table.
- Density altitude: Hot, humid, high-altitude air reduces power.
- Aerodynamics: Matters more at higher speeds.
- Drivetrain loss: Use wheel HP for better accuracy, not crank HP.
Trap Speed vs ET
Trap speed (the speed you cross the finish line at) is a better indicator of raw power than ET. ET is heavily influenced by the launch and 60-foot time, while trap speed reflects the car's average power output down the track. Two cars with the same trap speed have similar power — even if their ETs differ because one launched better.
1/8 Mile Estimates
The 1/8 mile values use standard conversion ratios from quarter-mile data. The 1/8 mile ET is roughly 63% of the quarter-mile ET, and the speed is roughly 81% of the trap speed. These are approximations.
Frequently Asked Questions
How do you estimate quarter-mile time from HP and weight?
The most common formula is the Bretterton-Ward equation: ET = 5.825 × (weight / HP)^(1/3). For trap speed: MPH = 234 × (HP / weight)^(1/3). These assume good traction and a competent driver. Real-world times depend heavily on launch, traction, and shifting.
What is trap speed and why does it matter?
Trap speed is the vehicle's speed at the finish line (1320-foot mark). It directly correlates with engine power — a higher trap speed means more horsepower regardless of how well you launched. ET (elapsed time) depends on both power and traction/launch. Two cars with the same trap speed have similar power even if their ETs differ.
How do 1/8 mile times relate to quarter-mile times?
A rough conversion: quarter-mile ET ≈ 1/8 mile ET × 1.55, and quarter-mile trap speed ≈ 1/8 mile trap speed × 1.25. These are approximations — the exact ratio depends on how much power the vehicle makes at higher speeds (aerodynamics and gearing matter more in the second half).
Related Articles
How quarter-mile elapsed time and trap speed work, what determines your ET, and the most effective ways to go faster — from weight reduction to traction to power adders.
GeneralWeight Transfer in Drag Racing: How to Get More Traction at LaunchHow weight transfer works during acceleration, what determines how much weight shifts to the rear, and how to use it to improve traction off the line.
GeneralPower-to-Weight Ratio: Why It Matters More Than Peak HorsepowerWhy power-to-weight ratio is the best single predictor of acceleration, how to calculate it, where your car sits relative to common benchmarks, and whether to add power or subtract weight.
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