Calculate your predicted 1/4 and 1/8 mile ET and trap speed from horsepower and weight, estimate power from a run, and project ET after car changes.
Drag Racing ET Predictor Formula
ET = C * (W / P)^(1/3)
MPH = K * (P / W)^(1/3)
ET_new = ET_base * ((W_new / W_old) * (P_old / P_new))^(1/3)
- ET is the predicted 1/4-mile elapsed time in seconds
- MPH is the predicted 1/4-mile trap speed in miles per hour
- W is the vehicle weight including the driver in pounds
- P is the engine power in horsepower
- C and K are empirical constants: Hale uses C = 5.825 and K = 234, Fox uses C = 6.269 and K = 230, Huntington uses C = 6.290 and K = 224
- ET_new is the projected elapsed time after a weight or power change, scaled from a real baseline run (ET_base)
The first two formulas power the main prediction mode: enter horsepower and race weight and the calculator returns 1/4-mile ET and trap speed, plus 1/8-mile figures using an ET factor of 1.56 and a speed factor of 1.26. The horsepower mode solves the ET equation backward, so a real ET and weight give the power the run implies. The adjustment mode uses the third formula to project a new ET from your actual baseline run after you add or remove weight or change power, which is more accurate for dial-in planning than a generic estimate because it starts from how your car really performs. The conversion mode applies the 1.56 and 1.26 factors directly to switch a slip between 1/8-mile and 1/4-mile tracks.
Power-to-Weight Benchmarks and ET Adjustment Rules
The table below shows typical 1/4-mile results at common power-to-weight ratios using Hale’s constants. Divide race weight (with driver) by horsepower to find your row.
| Power-to-weight (lb/hp) | Typical 1/4-mile ET | Typical trap speed |
|---|---|---|
| 4 | 9.25 s | 147 mph |
| 6 | 10.58 s | 129 mph |
| 8 | 11.65 s | 117 mph |
| 10 | 12.55 s | 109 mph |
| 12 | 13.34 s | 102 mph |
| 15 | 14.37 s | 95 mph |
| 20 | 15.81 s | 86 mph |
These rules of thumb show how common changes move the ET of a car running around 12 seconds. They come from the cube-root scaling formula, so larger or smaller cars shift proportionally.
| Change | Approximate ET effect |
|---|---|
| Remove 100 lb (3,500 lb car) | About 0.11 s quicker |
| Add 10 hp (400 hp car) | About 0.10 s quicker |
| Add 25 hp (400 hp car) | About 0.24 s quicker |
| Add a 180 lb passenger (3,500 lb car) | About 0.20 s slower |
How to Predict Drag Racing ET
Example one: predict the ET of a 3,400 lb car (with driver) making 450 horsepower. The power-to-weight ratio is 3400 / 450 = 7.56 lb/hp. Using Hale’s formula, ET = 5.825 * (7.56)^(1/3) = 11.43 seconds, and trap speed = 234 * (450 / 3400)^(1/3) = 119.3 mph. On an 1/8-mile track the same car predicts 11.43 / 1.56 = 7.33 seconds at about 94.6 mph.
Example two: project a new ET after removing weight. A car with a baseline of 12.20 seconds at 3,600 lb sheds 150 lb, giving a new weight of 3,450 lb. ET_new = 12.20 * (3450 / 3600)^(1/3) = 12.03 seconds, a gain of about 0.17 seconds without touching the engine.
FAQ
Why is my real ET slower than the prediction?
The formulas assume strong traction, a well-tuned launch, and near-sea-level air. Wheelspin, a slow 60-foot time, high density altitude, or a conservative shift strategy all add time. If your trap speed matches the prediction but your ET does not, the power is there and the launch is costing you; if both are down, the car is making less power than entered.
Which formula should I choose?
Hale’s constants were developed from quicker, better-hooking cars and suit modern performance and race vehicles. Fox and Huntington predict slower ETs and often fit heavier street cars on street tires. If you are unsure, run the average of all three and treat the spread as your expected range.
What weight and horsepower should I enter?
Use race weight: the car as it stages, with the driver and the fuel load it actually carries. For horsepower, flywheel figures line up best with the classic constants; if you only have a wheel dyno number, expect the prediction to be slightly slow, since drivetrain loss means the flywheel figure is roughly 10 to 15 percent higher.
