Quarter Mile Calculator
Predict 1/4-mile elapsed time (ET) and trap speed from weight and horsepower—or back-calculate horsepower from a time slip. Includes optional 1/8-mile conversions and clear, documented formulas.
Calculator
We’ll convert internally.
Common empirical sets from drag-racing references. Use “Custom” to tweak constants to your car.
Default 5.825
Default 234
Curb weight with driver, fuel, and typical track trim.
If you have wheel HP, add drivetrain loss (e.g., +12%) to approximate flywheel HP.
We’ll estimate 1/4-mile ET ≈ 1/8 ET × 1.56 (rule of thumb).
Results
Estimated ET (1/4-mile)
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Trap Speed (MPH)
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Estimated HP
—
Power-to-Weight
—
1/8-mile conversion & notes
Track prep, DA (density altitude), gearing and traction can shift these by noticeable margins; use your slips to refine constants.
Authoritative data source & methodology
NHRA — “NHRA 101” (definitions of elapsed time and speed measurement in the 66-ft trap at the finish).
Official reference.
Widely used empirical relations for street/strip estimates:
- Hale constants: \( ET = 5.825\,\big(\tfrac{W}{HP}\big)^{1/3} \), \( MPH = 234\,\big(\tfrac{HP}{W}\big)^{1/3} \)
- Alternative sets (Fox, others) vary slightly in constants (e.g., 6.269 / 230)
Tutti i calcoli si basano rigorosamente sulle formule e sui dati forniti da questa fonte.
The formula explained
Primary (Hale) relations
$$ ET = k_{ET}\,\left(\frac{W}{HP}\right)^{1/3}, \qquad MPH = k_{MPH}\,\left(\frac{HP}{W}\right)^{1/3} $$
Default constants: \(k_{ET}=5.825\), \(k_{MPH}=234\). Choose alternative sets or customize for your combo.
HP back-calculation from ET and weight:
$$ HP \approx W \left(\frac{k_{ET}}{ET}\right)^3 $$
1/8-mile heuristic (rule of thumb): \( ET_{1/4} \approx 1.56 \times ET_{1/8} \).
Glossary of variables
| Symbol | Meaning | Units |
|---|---|---|
W | Total race weight (car + driver + fuel) | lb (or kg) |
HP | Engine flywheel horsepower | hp (or kW) |
ET | Elapsed time for 1/4-mile | seconds |
MPH | Trap speed at the finish line | mph (or km/h) |
kET, kMPH | Empirical constants | — |
How it works: a step-by-step example
Goal: Estimate ET and trap speed for a 3,600 lb car making 400 hp (Hale constants).
- Compute \( \frac{W}{HP} = \frac{3600}{400} = 9 \). Cube root \(= 9^{1/3}\approx 2.0801\).
- \( ET = 5.825 \times 2.0801 \approx 12.12 \) s.
- Compute \( \frac{HP}{W} = \frac{400}{3600} = 0.111\overline{1} \). Cube root \(\approx 0.4807\).
- \( MPH = 234 \times 0.4807 \approx 112.5 \) mph.
Frequently asked questions
Do these constants match every car?
No. They’re averages for typical street/strip cars. Aero, gearing, traction, weather and driver can shift results. Use “Custom constants” to fit your slips.
Wheel vs flywheel HP?
Formulas assume flywheel HP. If you only have wheel HP, divide by (1 − driveline loss) to estimate flywheel (e.g., wheel 350 hp at 12% loss → flywheel ≈ 398 hp).
What exactly is “trap speed”?
The average speed over the final 66 ft of the run, measured at the finish (per NHRA timing rules).
My 1/8 and 1/4 times don’t match the 1.56 rule
That ratio is a heuristic; cars with strong back-half power or poor 60-ft will deviate.
Can I model DA or track prep?
Not directly here; use custom constants as a proxy or compare against your actual slips to tune.
Tool developed by Ugo Candido. Content reviewed by CalcDomain Editorial Board.
Last accuracy review: