1/8 mile calculator
To turn a 1/8-mile time into a quarter-mile time, multiply it by about 1.56. For trap speed, multiply by about 1.25. So a 7.00-second eighth at 100 mph points to about a 10.92-second quarter at 125 mph. The calculator does that both ways, estimates a car’s eighth mile from its weight and horsepower, and works out horsepower from an eighth-mile time slip.
Estimate an eighth mile
1/8 mile
6.99 s @ 99.6 mph
BMW M4 Competition xDrive (2022), simulated on a dry street.
- 0-60 mph2.80 s
- 60 ft1.58 s
- Quarter mile10.94 s @ 124.8 mph
- Car and Driver tested (street)11.0 s @ 125 mph
Classic formulas, from weight and power only
- Hale (1986), 1/8 mile (÷ 1.56, ÷ 1.25)7.50 s @ 93.2 mph
- Hale (1986), quarter mile11.69 s @ 116.6 mph
Estimates for a good launch and clean shifts on a mild day. Real runs vary with the driver, the weather and the track.
1/8 mile: 6.99 s @ 99.6 mph
The spec-sheet curb weight plus about 165 lb for you. Power is the rated figure at the engine (not at the wheels), or the motors’ combined rating for an EV.
How the conversion works
A quarter-mile time is about 1.56 times the eighth-mile time, and the quarter-mile trap speed about 1.25 times the eighth-mile speed. These are drag racers’ rules of thumb, and the people who publish them print slightly different numbers:
| Source | ET | Trap speed |
|---|---|---|
| DrivingLine (2021) | × 1.57, or 1.56 “in some instances” | × 1.25, 1.26 or 1.27 |
| calcHorsePower | × 1.557 | × 1.25 |
| ChevelleStuff | × 1.56 | – |
| CalculatorLib | × 1.5455 | × 1.255 |
| Wallace Racing | Varies with the time: 8.00 s gives 12.75 s | – |
The calculator uses 1.56 and 1.25. 1.56 sits in the middle of the published flat factors, and several sources print it or round to it; 1.25 is the speed factor nearly all of them give. None of them says which runs its factor came from. calcHorsePower explains why speed converts so steadily: it ties trap speed to the cube root of distance, and the cube root of 2 is 1.26.
1/4-mile ET = 1/8-mile ET × 1.56
1/4-mile trap speed = 1/8-mile trap speed × 1.25
| 1/8-mile ET | 1/4-mile ET, about |
|---|---|
| 4.50 | 7.02 |
| 5.00 | 7.80 |
| 5.50 | 8.58 |
| 6.00 | 9.36 |
| 6.50 | 10.14 |
| 7.00 | 10.92 |
| 7.50 | 11.70 |
| 8.00 | 12.48 |
| 8.50 | 13.26 |
| 9.00 | 14.04 |
From weight and horsepower, the calculator runs the car down the strip, as the quarter mile calculator does, and reads its time and speed at 660 feet. For comparison it shows Hale’s quarter-mile formula, and the same brought back to the eighth with the factors. From an eighth-mile slip it works out horsepower by turning the slip into a quarter mile first, then using Hale’s and Fox’s formulas (Jeff Lucius on the formulas). You can also run any car on our 3D drag strip.
The simulation runs the car down the strip a thousandth of a second at a time: its torque through the gears, how hard its tires can push with the weight that shifts onto them, the launch, each shift, rolling resistance and air drag. Pick a car from the list and it uses that car’s full spec, fitted to its road test. Enter your own and it builds a typical car of that kind around your weight and power. Times start once the car has rolled its first foot, as on a drag strip and in magazine tests, and the trap speed is the average over the last 66 feet.
Checking the factor on real cars
As our own check, not a source: we ran the 11 road-tested cars in our simulation, each fitted to its published quarter-mile test to within 3%, and compared each car’s simulated quarter mile with its eighth. The quarter-mile time came out 1.52 to 1.58 times the eighth-mile time, 1.54 on average, and the trap speed 1.22 to 1.27 times, 1.25 on average.
| Car | 1/8 mile | 1/4 mile | ET ratio | mph ratio |
|---|---|---|---|---|
| Cybertruck AWD | 8.02 @ 89.7 | 12.42 @ 111.1 | 1.549 | 1.239 |
| Cyberbeast | 6.96 @ 97.8 | 11.03 @ 119.6 | 1.585 | 1.223 |
| Model Y Long Range | 7.99 @ 92.1 | 12.26 @ 114.9 | 1.534 | 1.248 |
| Model Y Performance | 7.47 @ 97.6 | 11.49 @ 121.9 | 1.539 | 1.249 |
| BMW M2 (auto) | 7.55 @ 98.0 | 11.54 @ 123.8 | 1.529 | 1.263 |
| BMW M2 (manual) | 8.01 @ 94.5 | 12.15 @ 120.1 | 1.518 | 1.271 |
| BMW M3 Competition | 6.99 @ 99.0 | 10.96 @ 124.0 | 1.568 | 1.252 |
| BMW M4 Competition | 6.99 @ 99.6 | 10.94 @ 124.8 | 1.565 | 1.253 |
| Model S Plaid | 6.07 @ 120.4 | 9.33 @ 150.4 | 1.538 | 1.249 |
| Rimac Nevera | 5.43 @ 138.8 | 8.25 @ 175.0 | 1.518 | 1.261 |
| Honda Civic Sport | 11.07 @ 68.0 | 16.80 @ 86.1 | 1.518 | 1.267 |
More calculators
Tested times and mods, car by car
Questions
What is the 1/8 to 1/4 mile conversion?
Multiply the eighth-mile ET by about 1.56 and the eighth-mile trap speed by about 1.25. To go the other way, divide a quarter-mile ET by 1.56 and its trap speed by 1.25. A 7.00-second eighth works out to about 10.92 seconds in the quarter.
Why does the conversion factor vary?
Cars cover the second half of the run differently. DrivingLine names gearing, power and, on heavy trucks, air drag: a car geared for the eighth can run out of gear before the quarter. calcHorsePower adds that very powerful cars still pulling hard at the eighth, and cars that struggle for grip off the line, stray furthest from the rule. The flat factors published run from 1.5455 to 1.57 for time and 1.25 to 1.27 for speed.
How accurate is the conversion?
It’s a rule of thumb: none of the sources says which runs its factor came from. As our own check, we ran the 11 road-tested cars in our simulation, each fitted to its test. Their quarter-mile times came out 1.52 to 1.58 times their eighth-mile times, 1.54 on average, and their speeds 1.22 to 1.27 times. For a 7.00-second eighth, that spread is a quarter mile anywhere from 10.62 to 11.09 seconds.
Is 660 feet the same as an eighth mile?
Yes. A mile is 5,280 feet, so an eighth of a mile is 660 feet (about 201 meters) and a quarter is 1,320 feet. A quarter-mile time slip lists the time and speed at 660 feet: that’s the eighth-mile result.
Can I work out horsepower from an eighth-mile time slip?
Roughly. The calculator turns the eighth-mile speed into a quarter-mile speed (× 1.25) and uses Hale’s formula, hp = weight × (mph ÷ 234)³, or does the same with the ET (× 1.56). Trap speed is the better guide, because a slow launch costs time but barely changes the speed. A 4,070 lb car, a BMW M4 Competition with its driver, at 100 mph in the eighth works out to about 620 hp at the engine.