Eighth Mile Calculator
Convert eighth-mile ET and MPH to quarter-mile equivalents.
Many tracks are eighth-mile, but quarter-mile times are the universal benchmark. This calculator converts your 1/8-mile ET and trap speed into estimated quarter-mile figures using established conversion factors.
1/8 to 1/4 Mile Conversion
These multipliers work well for most street and strip cars. Very high-powered or traction-limited cars deviate slightly, since the second half of the quarter mile depends on power and aero more than the launch.
How to Use This Calculator
- Enter your eighth-mile ET in seconds.
- Optionally enter eighth-mile trap MPH.
- Read the quarter-mile estimates.
Worked Example
Why Convert Eighth-Mile to Quarter-Mile
Many drag strips — especially smaller or street-legal venues — run an eighth-mile (660 ft) instead of the full quarter (1,320 ft), often for safety or space. But the quarter mile remains the universal benchmark for comparing cars and bench-racing. These conversion factors let you translate an eighth-mile timeslip into a comparable quarter-mile ET and trap speed without making a full pass.
When the Conversion Is Most Accurate
The 1.557 ET and 1.25 MPH multipliers work well for typical street and bracket cars that pull cleanly through the traps. They drift for two kinds of cars: very high-powered machines still accelerating hard at the eighth (which gain more in the back half), and traction-limited cars that struggle off the line. We'd treat the result as a close estimate, not a guaranteed quarter-mile number.
Eighth-to-Quarter Conversion Examples
| 1/8 ET | 1/8 Trap | Est. 1/4 ET | Est. 1/4 Trap |
|---|---|---|---|
| 7.0 s | 98 MPH | 10.9 s | 123 MPH |
| 8.5 s | 85 MPH | 13.2 s | 106 MPH |
| 10.0 s | 70 MPH | 15.6 s | 88 MPH |
Why the ET Multiplier Is 1.5571, Not 1.414
The quarter mile is exactly twice the distance of the eighth. If a car accelerated at a constant rate the whole way, distance would scale with the square of time. Doubling the distance would then multiply elapsed time by the square root of two — about 1.414. Every real time slip disagrees, and the reason it disagrees tells you something useful about the car.
1.5571 → what cars actually do
Cars don't accelerate at a constant rate. Aerodynamic drag rises with the square of speed, the engine climbs past its torque peak, and every upshift costs momentum. By the time a car crosses the eighth-mile beam it is accelerating a lot less hard than it was off the line, so the second half of the track takes proportionally longer than pure geometry predicts. The 1.5571 factor is the empirical average of that shortfall across thousands of real passes.
That immediately tells you where the conversion is least reliable. A car whose acceleration falls off unusually fast in the second half — heavily aero-limited, badly geared, or out of breath at the top of its rev range — will run a multiplier above 1.5571. A car that keeps pulling hard past the eighth, such as a well-geared high-power machine, comes in below it.
Where the 1.25 Trap Speed Factor Comes From
The MPH multiplier has an even cleaner derivation. Under roughly constant power, the work done accelerating a car scales with its kinetic energy, which scales with the square of speed. Since work is force times distance, speed ends up proportional to the cube root of distance traveled. Double the distance and you multiply speed by the cube root of two:
The closeness of 1.25 to the theoretical 1.26 is the reason trap speed converts more reliably than elapsed time. Speed is governed by power and weight, which don't change down the track, whereas elapsed time absorbs everything that went wrong at the launch. This is also why horsepower estimates work better from trap speed than from ET — the same principle behind the trap speed horsepower calculator.
The 60-Foot Time Decides Most of the Difference
Two cars with identical power and weight can be four tenths apart in the eighth mile purely on how they left the line. The 60-foot time captures that, and it is the first number experienced racers look at on a slip — before ET, before trap speed.
| 60-foot time | What it indicates | Typical setup |
|---|---|---|
| 2.2 s and up | Significant wheelspin or a very soft launch | Street tires, cold track, or a cautious driver |
| 2.0–2.2 s | Reasonable launch on street rubber | Stock suspension, all-season or summer tires |
| 1.8–2.0 s | Good street and strip result | Drag radials, sensible tire pressures |
| 1.6–1.8 s | Strong launch, traction under control | Sticky tires and suspension set up for weight transfer |
| 1.4–1.6 s | Prepared car on a prepared surface | Slicks, adjustable shocks, transbrake or clutch launch |
| Under 1.3 s | Purpose-built race chassis | Full tube chassis, wheelie bars, prepped track |
The working rule at the strip is simple. Each tenth you gain in the 60-foot is worth about a tenth and a half through the eighth, and around two tenths by the quarter. Speed you carry out of the launch compounds down the whole track. A car with a 2.2 second 60-foot isn't slow; it is leaving several tenths on the start line before the engine has been asked a single question.
This matters for the conversion because a bad 60-foot inflates the eighth-mile ET without changing what the car is actually capable of. The multiplier will then project that inflated number all the way out to the quarter, compounding the error rather than correcting it.
When to Distrust the Converted Number
| Situation | What happens to the estimate |
|---|---|
| Poor 60-foot from wheelspin | Both converted figures come out pessimistic; the car is quicker than the projection says |
| Car runs out of gear before the eighth | ET converts too optimistically, because the real quarter would need another shift |
| Very high power, low drag | Real multiplier falls below 1.5571; the projected quarter ET reads slow |
| Tall, heavy or aerodynamically poor vehicle | Real multiplier climbs above 1.5571; the projection reads optimistic |
| Electric vehicle | Least reliable case — torque falls away sharply at speed, so the second half behaves nothing like a combustion car |
| High density altitude | Both halves are slower, but the ratio between them holds reasonably well |
The electric vehicle row deserves emphasis. Both conversion factors were derived from combustion cars whose power stays roughly flat across the upper rev range. An electric drivetrain typically holds constant power only to a point, then falls away as it reaches its speed limit, which makes the second half of the track disproportionately slow. Eighth-mile figures from an EV routinely project a quarter-mile ET that the car can't achieve.
Frequently Asked Questions
Multiply the eighth-mile ET by about 1.557. For example, an 8.5-second eighth-mile is roughly a 13.2-second quarter-mile.
Multiply the eighth-mile trap speed by about 1.25. So 85 MPH at the eighth becomes roughly 106 MPH at the quarter.
Eighth-mile tracks need less land and are safer for street cars and bracket racing, so they're very common, especially in some regions.
They're good estimates for typical cars, usually within a few tenths and a few MPH. High-power or low-traction cars can deviate more.
Yes — convert to quarter-mile figures first, then use the trap-speed or ET horsepower calculators. Convert the trap speed rather than the ET where you can, because trap speed reflects power and weight while ET absorbs everything that happened at the launch.
Because 1.414, the square root of two, assumes constant acceleration all the way down the track. Real cars accelerate progressively less hard as aerodynamic drag builds, the engine passes its torque peak and gears run out, so the second half takes longer than pure geometry predicts. The empirical 1.5571 figure captures that shortfall.
On street tires, 2.0 to 2.2 seconds is a respectable launch. Drag radials and a decent setup bring that to 1.8 to 2.0, prepared cars on slicks run 1.4 to 1.6, and full race chassis go under 1.3. As a working rule, each tenth gained in the 60-foot is worth about two tenths by the quarter-mile finish.
Poorly. Both factors were derived from combustion cars whose power stays roughly flat through the upper rev range. An electric drivetrain holds constant power only up to a point and then falls away as it approaches its speed limit, making the second half of the track disproportionately slow. Eighth-mile figures from an EV routinely project a quarter-mile ET the car can't actually run.