1/8 Mile Time Calculator

An eighth mile is 660 feet of track

An eighth mile is 660 feet, which is 0.125 mile or 201.168 meters. The number comes straight from the mile: 5,280 feet divided by eight is 660, and the international mile is exactly 1,609.344 meters per NIST Special Publication 811, so the eighth is exactly 201.168 meters. The exact factor sits in NIST's conversion tables. Neither figure is rounded.

The quarter mile is 1,320 feet, twice the distance, and that is the number most US tracks still print on the board. Plenty of strips run the eighth instead: it needs less shutdown area, it is easier on parts, and NHRA caps Jr. Dragster competition at 660 feet for the same reasons. If the schedule says 1/8 mile, every number on the slip is measured over those 660 feet, including the speed.

That matters when you compare slips from two tracks. A 7.90 at one strip and a 7.90 at another are the same distance only if both run the eighth. A 7.90 eighth mile and a 7.90 quarter mile are not close, and mixing them is the fastest way to get a useless answer out of any calculator, this one included. The distance box in the panel is there so you can check which race you are reading.

The three numbers on an eighth mile slip

A slip from a 660 foot pass carries three numbers that matter. The 60 foot time covers the launch. The 660 foot ET is the time from the starting beam to the finish beam. The 660 foot mph is the trap speed, averaged over a short window at the end of the run rather than measured at the instant the car crosses the line.

That last point explains most of the confusion in the pits. ET is a stopwatch over the whole distance, so a hard launch improves it. Trap speed measures how much speed the car carried through the lights, so it leans on power and gearing. A car that hooks up and leaves hard can beat a more powerful car to 660 feet and still trap 5 mph slower.

Read the two together and the slip tells you where the car is losing time. Quick ET with a low trap speed points to a strong launch and a lazy top end. Slow ET with a high trap speed points to wheelspin or a soft launch that the car never recovers from. Same ET, different problem, different fix.

Average speed and average acceleration from the ET

Two divisions turn the slip into physics. Average speed is distance divided by time: 660 feet divided by 8.00 seconds is 82.50 feet per second, and multiplying by 3,600 then dividing by 5,280 gives 56.25 mph. Average acceleration, assuming the car started from rest, is 2d/t squared, the form listed in OpenStax's constant acceleration equations, so 1,320 divided by 64.00 is 20.63 feet per second squared. Divide by 32.174, standard gravity in feet per second squared, and the run averaged 0.641 g.

Those are averages, and the gap between average and peak is the useful part. If acceleration stayed flat, the finish speed would be twice the average, or 165.00 feet per second, which is 112.50 mph. The slip says 85 mph. The car therefore did most of its acceleration early and spent the back half of the track trading acceleration for speed, which is what a traction limited launch looks like on paper.

The same arithmetic on a quicker pass: 7.00 seconds over 660 feet averages 94.29 feet per second, or 64.29 mph, with 26.94 feet per second squared of average acceleration, or 0.837 g. A flat acceleration model would predict 188.57 feet per second at the finish, or 128.57 mph. A 98 mph slip is closer to that ceiling than the 85 mph example was, which suggests the quicker car kept pulling harder for longer.

An ET to speed table for the eighth mile

The table below divides 660 feet by each ET, so every row follows the same arithmetic as the panel. 660 feet divided by 6.00 seconds is 110.00 feet per second, and 110.00 times 3,600 divided by 5,280 is 75.00 mph. The quarter column multiplies the ET by 1.57.

Turning 660 feet into a quarter mile estimate

Multiplying the eighth mile ET by 1.57 gives a quarter mile estimate, and multiplying the 660 foot trap speed by 1.25 gives a quarter mile trap speed. An 8.00 second eighth becomes 12.56 seconds, and 85 mph becomes 106.25 mph. The factor is not 2 because the car does not start the second 660 feet from a standstill. It enters that stretch already moving, so the second half takes less time than the first.

The factor is a trackside convention, not a law. Charts use 1.55 to 1.58 depending on who built them, and the spread on an 8.00 second pass is 12.40 to 12.64 seconds, about a quarter of a second. Cars that make their power upstairs, or are geared for the quarter, tend to run quicker than 1.57 predicts once they get the full 1,320 feet. Cars geared short for the eighth tend to run slower.

Use the answer as a planning number. It is good enough to pick a class, compare two builds, or decide whether a change is worth a test session. It is not a substitute for a quarter mile slip, and the panel says so under the result.

What the ET says about power to weight

The oldest empirical shortcut in drag racing is Hale's formula: quarter mile ET equals 5.825 times the cube root of race weight divided by horsepower, with weight in pounds. Run it backward and the ET becomes a power to weight reading. Divide the quarter ET by 5.825, cube the result, and you have pounds per horsepower.

For the 12.56 second estimate, 12.56 divided by 5.825 is 2.156, and 2.156 cubed is 10.02. At a 3,200 pound race weight that implies 319 horsepower. A quicker example: 7.00 seconds at 98 mph in a 2,800 pound car projects to 10.99 seconds, which is 6.72 pounds per horsepower, or 417 horsepower. A slower one: 9.50 seconds in a 3,600 pound car projects to 14.92 seconds, which is 16.79 pounds per horsepower, or 214 horsepower.

Treat those numbers as flywheel estimates with a wide margin. The formula assumes a clean launch and a chassis that can put the power down. A car with traction problems will look more powerful than it is, because the ET is slow for its weight. A car with a loose converter or a short tire will look less powerful, because the ET is quick without the trap speed to match.

Why two cars with the same ET trap 8 mph apart

Elapsed time and trap speed answer different questions, so they do not move together. Take two 8.00 second eighth mile cars. One leaves at 1.30 in the 60 foot and traps 78 mph, the other leaves at 1.42 and traps 90 mph. The first car won the first 60 feet and ran out of breath. The second gave away a tenth at the launch and made it back with top end power.

Gearing is the usual cause. A short gear multiplies torque at the wheels, which helps the launch and hurts the speed at 660 feet. A tall gear does the opposite. Converter stall, tire diameter and shift points all move the balance between the two numbers, which is why a converter change can pick up ET and lose trap speed in the same pass.

Air matters too. Density altitude changes both numbers on the same car, and the effect is larger on a naturally aspirated engine than on a boosted one. Comparing a morning pass at 500 feet of density altitude with an afternoon pass at 3,000 feet will show a difference that has nothing to do with the tune. Keep the conditions with the slip if you plan to compare runs later.

The same split, laid out side by side:

Using the panel without fooling yourself

Check the distance box first. 660 is the eighth and 1,320 is the quarter. Check the trap speed too: if the slip says 660 mph, that is the number to type, not the quarter mile mph from a different run. Mixing the two is the most common way to get a quarter mile estimate that looks fast and means nothing.

The two factor boxes are editable on purpose. If your track or your class uses a chart with 1.56, type 1.56 and the projection moves with it. If you are reading a quarter mile slip and want the eighth mile it implies, set both factors to 1 and enter 1,320 feet, then divide the result by 1.57 yourself, or set the ET factor to 0.637 and the speed factor to 0.80.

Last, compare like with like. A pass at sea level on a cool morning and a pass at 3,000 feet on a hot afternoon are not a fair test of a change. Run the calculator on each slip, write the conditions next to the result, and only then decide whether the new part helped.