Work forwards or backwards from your target
Racing ET Calculator
Most calculators only go one way. This one lets you choose what to solve: ET from power and weight, power for a target ET, power from trap speed, or the weight limit that gets you to a given time.
- 1/8-mile ET
- 7.69 sec
- Weight per hp
- 8.75 lb/hp
Four ways to solve the same equation
The relationships are linked: ET = 5.825 × (W ÷ HP)1/3 and MPH = 234 × (HP ÷ W)1/3. Rearranged, HP = W × (5.825 ÷ ET)3, HP = W × (MPH ÷ 234)3 and W = HP × (ET ÷ 5.825)3. For a 3,500 lb car to run 11.5 s you would need about 455 hp at the crank.
The estimate uses the widely published Patrick Hale equations: ET = 5.825 × (weight ÷ hp)1/3 and MPH = 234 × (hp ÷ weight)1/3, with weight in pounds and hp measured at the crank. They are documented, along with the more conservative Huntington (6.290 / 224) and Fox constants, in Stealth316’s horsepower / ET / MPH calculator notes. See the full formulas and sources page.
Horsepower needed for a target ET (3,500 lb)
| Target ET | Crank hp needed | Trap speed |
|---|---|---|
| 14.00 s | 252 | 97.4 mph |
| 13.00 s | 315 | 104.9 mph |
| 12.00 s | 400 | 113.6 mph |
| 11.00 s | 520 | 123.9 mph |
| 10.00 s | 692 | 136.3 mph |
| 9.00 s | 949 | 151.5 mph |
Maximum weight for a target ET (400 hp)
| Target ET | Max race weight |
|---|---|
| 13.00 s | 4,446 lb |
| 12.00 s | 3,497 lb |
| 11.00 s | 2,694 lb |
| 10.00 s | 2,024 lb |
Take these as the direction of travel: real cars rarely hit the idealized numbers.
Frequently asked questions
How much horsepower do I need to run an 11 second quarter mile?
A 3,500 lb car needs roughly 520 hp at the crank by the formula. A 3,000 lb car needs about 445 hp.
How do I estimate horsepower from trap speed?
Use HP = weight × (trap MPH ÷ 234)³. Trap speed is a better indicator of power than ET because it is less affected by launch.
Why can ET and MPH disagree?
If ET looks slower than the trap speed suggests, the car is likely losing time at the launch (traction or reaction). That is the ET and MPH mismatch that is useful to tune.
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