Instrument № 03 - Speed

Quarter Mile
ET & Trap

Estimate elapsed time and trap speed from weight and horsepower - with corrections for drivetrain loss, altitude, traction, and rollout. Or work the math backwards: what power do you need to hit a target?

See the math behind this calculator

Distance
1,320 ft / 402 m

ET formula constant
5.825 (Fox)

Driveline loss typical
10 – 20%

Units

Vehicle

Weight (lb) - include driver & fluids

Power figure

Power source

Drivetrain loss (%)

Conditions

Altitude power loss (%)

Traction quality

1-foot rollout

Formula coefficient

Reverse solver - power for a target

Target ET (seconds)

Target trap (mph)

Estimated ET -

Trap speed -

Effective wheel power -

Power-to-weight -

Need to hit ET target -

Need to hit trap target -

The math

Two old empirical formulas have predicted quarter-mile times for decades. They both depend on a single ratio: power divided by weight.

ET ≈ 5.825 × ∛(weight ÷ WHP)
trap ≈ 234 × ∛(WHP ÷ weight)

Wheel horsepower is the figure that matters. Crank numbers from a marketing sheet need to be discounted by the drivetrain - typically 10–25% depending on transmission and driveline.

Frequently asked

Why isn’t my real ET matching the calculator?

Calculator outputs assume a clean launch, sticky surface, and that the formula constants describe your car. Real-world runs are dragged down by wheelspin off the line, slow shifts, narrow tires, ethanol-free fuel choices, hot air, and altitude. A street tire alone can add half a second.

Should I use the “1-foot rollout” setting?

Yes if you’re comparing to drag-strip timeslips. NHRA timing starts 1 foot past the staging beam, which makes ETs about 0.3 seconds quicker than a true zero-roll measurement. Off-strip dyno-style comparisons should use rollout off.

What traction setting fits a stock daily driver?

Average street. Sticky is for drag radials on prepped concrete. Poor is for snow tires or rain. Very poor is for a smoky burnout - useful mostly to see how punishing wheelspin actually is.

Show the working

These are the two classic empirical drag racing estimators. They are curve fits to observed results, not physics derived from first principles, which is why two different coefficient sets exist. If any of it looks wrong, it might be - tell us what we got wrong.

Elapsed time ET = k_ET × ∛(weight ÷ wheel horsepower)

Trap speed MPH = k_MPH × ∛(wheel horsepower ÷ weight)

Adjustments applied to ET

ET = ET × traction factor
with rollout → ET = ET − 0.30

Crank power converted to wheel power whp = bhp × (1 − drivetrain loss)

Power required to hit a target

from a target ET  → whp = weight ÷ (target ET ÷ k_ET)³
from a target MPH → whp = weight × (target MPH ÷ k_MPH)³
crank = whp ÷ (1 − altitude loss) ÷ (1 − drivetrain loss)

Constants used

k_ET 5.825, k_MPH 234 — the standard Roger Huntington coefficients
k_ET 6.269, k_MPH 225 — the alternative Patrick Hale coefficients
0.30 seconds — typical rollout credit on a Pro tree
15% — default drivetrain loss from crank to wheels

Both estimators assume a competent launch and a car that hooks up. They describe a well-sorted vehicle on a prepped surface, so treat the output as a ceiling rather than a prediction of your next run.