Bidirectional ET conversion with trap speed, vehicle-type factors, Hale’s horsepower cross-check & density-altitude correction.
Units
Switching converts trap speed, weight, and density-altitude inputs. ET stays in seconds; power can be hp or PS.
Direction
Required
Optional — trap speed
Optional — Hale’s cross-check (weight + power)
Optional — density altitude
Results — conversion is an estimate, ±0.15 s typical
What this means
Share & saved setups
Setups are stored in this browser only (localStorage).
Worked examples
1/4-mile ET: 7.50 × 1.57 = 11.78 s (typical range 11.63–11.93 s).
1/4-mile trap: 95 × 1.26 = 119.7 mph (192.6 km/h).
Hale’s check (3,600 lb, 500 hp): ET 12.11 s, trap 119.1 mph — within 0.33 s / 0.5 mph of the conversion.
DA 3,000 ft (rough): ET 12.13 s, trap 117.2 mph.
1/8-mile ET: 10.20 ÷ 1.54 = 6.62 s (typical range 6.47–6.77 s).
1/8-mile trap: 132 ÷ 1.26 = 104.8 mph (168.6 km/h).
Hale’s check (700 lb bike + rider, 180 hp): ET 9.86 s — about 0.34 s quicker than the conversion, a normal disagreement for bikes (see Limitations).
Tip: type either example into the inputs above and hit Convert — you should get the same numbers.
FAQ
Why is multiplying by 1.57 only an estimate?
The factor is an empirical average from thousands of time slips. How hard a car pulls on the top end depends on gearing, aero, and power curve — a car that spins early but pulls hard up top converts “better” than 1.57, and vice versa. Treat the result as a ±0.15 s window, not a promise.
What does “±0.15 s typical” mean in practice?
Most similar vehicles land within 0.15 seconds of the converted ET. So 7.50 × 1.57 = 11.78 really means “expect roughly 11.63–11.93”. If your actual slip falls outside that window, your car’s power delivery simply differs from the average — use your own slips from then on.
Why do motorcycles and dragsters use different factors?
Motorcycles (×1.54) and Top Fuel dragsters (×1.53) accelerate violently early and are drag-limited late, so proportionally more of the run happens in the first 1/8. Door cars (×1.57) keep pulling harder through the back half.
What is density altitude, and what does the correction do?
Density altitude (DA) combines elevation, temperature, and humidity into “how thin the air feels to the engine.” Thin, hot air makes less power: ETs rise and trap speeds fall. The toggle applies a rough rule of thumb (≈1% ET per 1,000 ft DA) — coarse, but useful for comparing a mountain pass to a sea-level pass.
How does the Hale’s cross-check work?
Hale’s formulas (ET = 6.269 × (weight/hp)⅓, trap = 230 × (hp/weight)⅓) estimate quarter-mile performance from weight and power alone — completely independent of your time slip. If both methods agree, your numbers are consistent; if they disagree by a lot, something (traction, weather, a tired engine) deserves a look.
Limitations
- The 1.53 / 1.54 / 1.57 factors are empirical averages — individual cars vary with gearing, traction, and power delivery. They are worst for unusual combos (e.g. a turbo car that spins the whole 1/8).
- The DA correction is a rough rule of thumb, not NHRA’s correction tables or a weather-station calculation. It ignores humidity and track prep.
- Hale’s formulas were derived from 1980s stock cars. They tend to overestimate motorcycle trap speed (bikes have poor aerodynamics for their power-to-weight) and ignore launch traction entirely.
- ET is dominated by the launch (60-ft); two cars with identical quarter ETs can have very different 1/8-mile splits.
What a 1/8 Mile to 1/4 Mile Conversion Does
A 1/8 mile to 1/4 mile conversion estimates your quarter-mile elapsed time from an eighth-mile pass (or the reverse). Most small tracks run the eighth mile, but quarter-mile ETs are the standard people compare. This tool multiplies your ET by a vehicle-appropriate factor — 1.57 for a typical street car — to give a realistic quarter-mile estimate.
How to Use
- Enter your 1/8-mile elapsed time in seconds (e.g., 8.50).
- Pick a vehicle-type factor. Street cars use 1.57; motorcycles use 1.54; very quick drag cars use ~1.53.
- Read the estimated 1/4-mile ET. The result is an estimate, not a prediction — treat it as a ballpark good to within about 0.15 seconds.
- Reverse direction: enter a quarter-mile ET to estimate the corresponding eighth-mile time (divide by the same factor).
Formulas and Worked Examples
Eighth-mile to quarter-mile
Quarter-mile ET ≈ eighth-mile ET × 1.57
Example: you run the eighth mile in 8.50 seconds.
8.50 × 1.57 = 13.345 → ~13.35 s
Quarter-mile to eighth-mile (reverse)
Eighth-mile ET ≈ quarter-mile ET ÷ 1.57
Example: your car runs a 12.00-second quarter mile.
12.00 ÷ 1.57 = 7.643 → ~7.64 s
About the factor
Honestly: the factor isn’t fixed. It runs roughly 1.53–1.57 depending on the vehicle. A 1.57 factor suits a typical street car with a normal power curve. Purpose-built drag cars that pull hard all the way down the track land near 1.53, while slow or heavy vehicles that run out of steam sit near the top of the range (about 1.57). Pick the factor that matches your vehicle type, or treat 1.57 as the honest middle.
Common Mistakes
- Treating the estimate as exact. The conversion is a rule of thumb good to within a tenth or two. Traction, gearing, weather, and driver skill all move the real number — don’t bet money on the estimate.
- Using one factor for dragsters and street cars. A 4-second eighth-mile dragster and a 10-second eighth-mile sedan don’t scale the same way. The factor compresses toward 1.53 as trap speed and power-to-weight climb.
- Ignoring trap speed differences. Two cars can run identical eighth-mile ETs and very different quarter-mile ETs if one traps much faster. ET conversion without trap speed is inherently approximate — a car still accelerating hard gains more in the back half.
- Forgetting density altitude effects. A pass at 5,000 feet of density altitude is slower than the same car at sea level. Converting an ET without correcting for air density bakes that penalty into your estimate.
- Comparing corrected and uncorrected times. Some slips and calculators apply weather correction and some don’t. Know which one you’re entering before you compare your converted number against someone else’s.
Frequently Asked Questions
Why isn’t the conversion factor the same for every car?
Because the back half of a quarter mile isn’t a fixed percentage of the run. A car that launches hard but flattens out (traction-limited street car) spends relatively more time in the second half, so its factor runs higher (~1.56–1.57). A car that keeps pulling — big power, good aero, proper gearing — covers the second half faster relative to the first, pushing the factor toward 1.53. The factor encodes the shape of the acceleration curve, and that shape differs by vehicle.
Can you convert trap speed too?
Roughly, yes. Quarter-mile trap speed is typically about 1.2–1.3× the eighth-mile trap speed for a street car (e.g., 85 mph in the eighth ≈ 105–110 mph in the quarter). The relationship is looser than the ET one, because trap speed depends heavily on how the car accelerates in the back half. Treat any trap-speed conversion as a wider estimate.
How accurate is the estimate, really?
For a typical street car at similar conditions, within about ±0.15 seconds. The estimate assumes a normal power curve, decent traction, and comparable weather. It breaks down for unusual vehicles (electric cars with instant torque but limited top end, or very slow cars) and when comparing passes from very different density altitudes.
What is density altitude correction?
Density altitude combines elevation, temperature, humidity, and barometric pressure into a single number: the altitude the air “feels like” to your engine. High density altitude means thin air — less oxygen, less power, slower ETs. Drag racing uses correction factors (from NHRA/SAE tables) to normalize ETs to sea-level conditions. If you’re converting an eighth-mile ET run on a hot day at elevation, correct for density altitude first for an honest comparison.
Why do some tracks only run the 1/8 mile?
Mostly shutdown room and insurance. A quarter mile needs a long, safe shutdown area after the finish line — many smaller or older tracks physically don’t have it. The eighth mile also suits bracket racing and test-and-tune nights: shorter runs mean less wear, quicker turnaround, and lower speeds that keep insurance costs down. It’s a practical constraint, not a racing preference.