1/8 Mile calculator

Why 1/8 Mile Numbers Get Quoted So Much at the Track

Walk into any bracket racing event and you'll hear eighth-mile times thrown around more than quarter-mile. Shorter tracks, faster turnaround between runs, and less risk of something going wrong at higher speeds make the eighth mile the default for a lot of grassroots drag racing.

Your elapsed time and trap speed at the eighth mile aren't simply half of your quarter-mile numbers. Acceleration isn't linear, a car is still building speed hard at the 660-foot mark, so the eighth-mile trap speed comes in noticeably lower than half the quarter-mile trap speed would suggest.

The Weight-to-Power Formula That Actually Works

Drag racing calculators rely on a formula developed from decades of recorded runs: ET = 5.825 × (weight ÷ horsepower)^(1/3). This was built for the quarter mile originally, and the eighth-mile figure comes out to roughly 64% of that quarter-mile time for most naturally aspirated vehicles.

Here's a worked example. A 3,400 lb car with 320 horsepower: weight-to-power ratio is 10.625. Cube root of that is about 2.196. Multiply by 5.825 and you get a 12.79 second quarter mile. Take 64% of that and the eighth-mile ET lands around 8.19 seconds, with a trap speed near 81 mph.

Trap speed at the eighth mile uses a similar relationship: 234 × (hp ÷ weight)^(1/3), then scaled to roughly 79% of the full quarter-mile trap speed since the car hasn't reached its terminal velocity yet.

Where the Formula Breaks Down

This math assumes a clean launch, good traction, and a naturally aspirated engine. Add a turbo or supercharger and the power curve changes shape entirely, the formula will undershoot your real numbers because boosted cars build power differently through the rev range than the data set this formula was built on.

Tire compound matters more than most people think. Drag radials versus street tires on the same car can produce a half-second swing in eighth-mile ET purely from launch traction, nothing to do with the engine at all.

• Naturally aspirated, good traction: formula is usually within 0.2-0.3 seconds of a real run
• Turbocharged or supercharged: expect the formula to undershoot actual performance
• Poor traction or a bad launch: real ET will be slower than predicted, sometimes by a full second

Converting Between 1/8 and 1/4 Mile

If you've already got a quarter-mile time slip and want the eighth-mile equivalent, the conversion calculator mode above skips the weight and horsepower math entirely and just applies that 0.64 ratio directly to your known ET. It's less precise for predicting trap speed since that scales differently, but for elapsed time alone it holds up well across most stock and mildly modified cars.

Reaction time isn't part of this calculator at all, since that's purely about driver response off the tree and has nothing to do with vehicle physics. A 0.015 second reaction time and a 0.4 second one will show identical eighth-mile ETs on the timing slip's "ET" line, the difference only shows up in your total elapsed time from the tree.

FAQs

How do I calculate 1/8 mile time from horsepower and weight?

Use the formula ET = 5.825 × (weight ÷ horsepower)^(1/3) to get the quarter-mile time, then multiply by roughly 0.64 to estimate the eighth-mile equivalent. This calculator does both steps automatically.

What is a good 1/8 mile time for a stock car?

Most stock daily-driven sedans run somewhere between 9.5 and 11 seconds in the eighth mile. Sports cars and modified vehicles can dip into the 7-8 second range or quicker.

How accurate is the 1/8 mile to 1/4 mile conversion calculator?

It's reasonably accurate for naturally aspirated vehicles with consistent traction, typically within a couple tenths of a second. Turbocharged cars and inconsistent track conditions widen that margin.

Does 1/8 mile trap speed calculator account for tire type?

No, the weight-to-power formula assumes average traction conditions and doesn't factor in specific tire compounds. Drag radials or slicks will produce a faster real-world trap speed than the formula predicts.

Why is eighth mile trap speed not exactly half of quarter mile trap speed?

Acceleration builds progressively, so a car is still gaining speed quickly at 660 feet rather than having already reached its peak. That's why eighth-mile trap speed comes out to around 79% of quarter-mile trap speed rather than 50%.

Can I use this horsepower from 1/8 mile calculator in reverse?

This version is built to go from weight and horsepower to time, not the reverse. Estimating horsepower from a known ET requires a different formula and introduces more uncertainty, since weight, traction, and gearing all affect the result.

Is reaction time included in the 1/8 mile elapsed time calculator?

No, this calculates pure vehicle performance from the moment of launch, not your reaction off the starting tree. Reaction time is a separate driver-skill metric tracked independently on a timing slip.

Take any prediction here as a solid starting estimate rather than gospel, the only way to know your real number is a timing light and an actual run down the strip.