Quarter Mile Calculator (ET, Trap Speed & Horsepower)

Estimate 1/4 mile elapsed time, trap speed, and horsepower from weight, power, or time. Includes 1/8 mile conversions and drivetrain loss.

1. ET & Trap from Weight & Horsepower

Include driver, fuel, and cargo (race weight).

%

Set to 0% if you entered wheel HP.

Used to slightly adjust traction assumptions.

How this quarter mile calculator works

This tool combines several popular drag racing formulas to estimate elapsed time (ET), trap speed, and horsepower. It is designed for street cars, bracket cars, and typical sports cars running the 1/4 mile.

1. ET from weight and horsepower

A widely used empirical relationship between race weight and wheel horsepower is:

Quarter mile ET (seconds)

\[ \text{ET}_{1/4} \approx 5.825 \times \left(\frac{W}{P_\text{wheel}}\right)^{1/3} \]

where:

  • \(W\) = race weight in pounds (car + driver + fuel)
  • \(P_\text{wheel}\) = wheel horsepower (whp)

If you only know crank horsepower, the calculator converts it to wheel horsepower using your drivetrain loss setting:

\[ P_\text{wheel} = P_\text{crank} \times (1 - \text{loss}) \]

Example: 400 hp at crank, 15% loss → \(400 \times 0.85 = 340\) whp.

2. Trap speed from weight and horsepower

Trap speed is less sensitive to traction and launch, so it often reflects power more reliably than ET:

Quarter mile trap speed (mph)

\[ V_{1/4} \approx 234 \times \left(\frac{P_\text{wheel}}{W}\right)^{1/3} \]

The calculator applies a small adjustment based on drivetrain type (FWD, RWD, AWD) to reflect typical traction differences, but the dominant factors are still weight and power.

3. Horsepower from ET and weight

Rearranging the ET formula gives an estimate of wheel horsepower from a known ET and race weight:

\[ P_\text{wheel} \approx \frac{W}{\left(\dfrac{\text{ET}_{1/4}}{5.825}\right)^3} \]

The calculator then estimates crank horsepower using your drivetrain loss:

\[ P_\text{crank} = \frac{P_\text{wheel}}{1 - \text{loss}} \]

4. 1/8 mile ↔ 1/4 mile conversions

Many tracks run 1/8 mile only. A common rule of thumb to convert 1/8 mile ET to 1/4 mile ET is:

\[ \text{ET}_{1/4} \approx \text{ET}_{1/8} \times 1.57 \]

\[ V_{1/4} \approx V_{1/8} \times 1.25 \]

These factors are averages; some cars pick up more mph on the back half than others depending on gearing, aerodynamics, and power curve.

Getting the most accurate estimates

Use true race weight

  • Include driver, helmet, fuel, and any tools or cargo you actually run with.
  • If you do not know race weight, add 200–250 lb to curb weight as a rough estimate.

Wheel HP vs crank HP

  • If you have a dyno sheet, use wheel horsepower and set drivetrain loss to 0%.
  • If you only know crank HP, use a realistic loss:
    • FWD manual: ~10–15%
    • RWD manual: ~12–18%
    • Automatic or AWD: ~18–25%+

Understand the limitations

These formulas assume:

  • Good traction and a reasonably optimized launch.
  • Average gearing and shift times.
  • Sea-level conditions and typical street/strip aerodynamics.

If your car spins off the line, bogs, or you lift early, your actual ET will be slower than the estimate even if the trap speed is close.

FAQ

How accurate is this quarter mile calculator?

For typical street and mild track cars, it is often within a few tenths of a second and a few mph when you use realistic race weight and wheel horsepower. Highly optimized drag cars, very poor traction, or extreme aero setups can deviate more.

Can I use this for motorcycles or very light cars?

The formulas were derived mainly from cars in the 2,000–4,000 lb range. They can still give a ballpark for lighter vehicles, but expect more error. For bikes and ultra-light cars, traction and aerodynamics dominate.

Why is my trap speed close but ET is slower?

That usually means your car makes the expected power, but the launch and 60-foot time are not optimized. Improving traction, tire choice, and launch technique can drop ET significantly without changing horsepower.

Does weather (DA) affect these estimates?

Yes. High density altitude (hot, humid, high elevation) reduces power and slows ET and mph. This calculator assumes average conditions; if you race at high DA, your real times will be slower than the theoretical sea-level estimate.