Quarter Mile Calculator

Predict your quarter mile ET and trap speed from power and weight - four published formulas side by side, each with its real-world accuracy. Or convert an eighth mile run without pretending one magic factor exists.

What do you have

Engine power

Power unit

Weight with driver

Weight unit

Where is your power number from

The formulas were fitted against factory crank ratings, so a rated figure goes straight in. A chassis dyno reads lower - converting it up needs an assumed drivetrain loss, and that is an assumption, not a measurement.

Formula	ET	R² of ET fit	Trap	R² of trap fit

Accuracy: these formulas assume a hard, clean launch on a prepped strip with period-typical tires. A modern street car on street tires usually runs slower than the ET shown, sometimes by half a second or more, because ET is dominated by launch and traction. The trap speed is the trustworthy half of this prediction - it depends almost only on power and weight. The constants are empirical, with typical light-car aero baked in, so bricks and very slippery cars drift off the fit, and wind alone moves trap speed 2 to 4 mph.

Factor source	ET factor	Quarter mile ET

Accuracy: there is no single correct conversion factor, which is why this table shows the published span instead of picking one. The factors are fleet averages: a car that leaves hard but noses over up top converts high, a car that charges in the second eighth converts low, and very fast cars fall below every factor here because aero drag eats the back half. If you have power and weight, the prediction mode above is the better estimate.

How the prediction works

Over a standing quarter mile, ET and trap speed both scale with roughly the cube root of power to weight - that part is physics. The constants are not: they were fitted to real runs. Huntington fitted 224 and 6.290 to late-1950s drag results (Rod & Custom, December 1958). Fox derived the relationship from first principles in 1973 in the American Journal of Physics and last updated his constants in 2001. Hale's 234 and 5.825 came with his Quarter and Quarter Jr software in 1986, fitted to a large strip database. The Road & Track best fit (Lucius, 2008) also freed the exponents and regressed against 243 magazine road tests, landing on the strongest fit of the four: R² 0.93 on trap speed and 0.91 on ET. Weight means as-raced weight including the driver, roughly curb weight plus 200 lb. The default car is the 3,900 lb, 320 hp Mitsubishi 3000GT VR4 worked example from that regression - the sibling horsepower from trap speed calculator runs the same math in reverse, so the two tools always agree.

Why your street car will run slower

The formulas were fitted to cars that launched well. ET is dominated by the first 60 feet: street tires, an unprepped surface, a cautious clutch or a lazy converter can add half a second that has nothing to do with power. Trap speed is different - by the top of the quarter mile the launch has washed out and speed is set almost entirely by power and weight, which is why the trap column fits real data at R² 0.91 to 0.93 while the ET column manages only 0.62 to 0.91. Read the prediction accordingly: the trap speed is a genuine estimate, the ET is a best case. The power figure should be a crank rating; a chassis dyno number reads lower, and the wheel-to-crank conversion this tool offers is a labelled assumption, not a measurement. Asking the opposite question - how much horsepower a 12 or 10 second quarter mile needs - is what the horsepower from trap speed calculator does, with the same four formulas run in reverse.

Eighth mile to quarter mile, honestly

Most published converters multiply your eighth mile ET by a single factor and stop. The problem: the published factors disagree. Widely used tables multiply by 1.555 or 1.56, Wallace Racing's calculator works out near 1.59, and the NHRA rule of thumb is 1.6. Physics puts a car with constant power and no launch phase at exactly 2^(2/3), about 1.587 - real cars land below that in proportion to how much of their run is launch. So this tool shows the span, 1.555 to 1.60, and tells you which end your car likely sits at. Trap speed converts more reliably: multiplying by 1.26 (2^(1/3), the same constant-power result) matches the commonly quoted street multiplier, with published values running 1.24 to 1.27.

Enter Your Car

Where is your power number from

Quarter Mile Prediction

Prediction Spread

Eighth Mile Conversion

How the prediction works

Why your street car will run slower

Eighth mile to quarter mile, honestly