How the Engine Horsepower Calculator Works
You can estimate the power of a car without a dyno, from what it does on the drag strip. In a quarter-mile run, the speed at the end (the trap speed) tells you how much power was pushing the car, because to go faster against the air and the weight of the car you need more power. This calculator turns the weight of the car and the trap speed into an estimate of the engine horsepower.
The formula
horsepower = weight x (trap speed / 234) x (trap speed / 234) x (trap speed / 234)
= weight x (trap speed / 234) cubed
The weight is in pounds and the trap speed is in miles per hour. The number 234 is a constant that was found from racing data. In the calculator you can also enter kilograms and kilometers per hour, which are changed to pounds and mph inside.
The speed is cubed because the power needed to go faster grows with the cube of the speed: it takes 8 times the power to go twice as fast against the air. The weight is not cubed, because a heavier car needs more power in a straight proportion.
Worked example: 3,200 lb at 105 mph
| Step | Calculation | Result |
|---|---|---|
| Speed ratio | 105 / 234 | 0.4487 |
| Cubed | 0.4487 x 0.4487 x 0.4487 | 0.09035 |
| Engine horsepower | 3,200 x 0.09035 | 289 hp |
| Power at the wheels with a 15% loss | 289 x 0.85 | 246 hp |
| Weight for each horsepower | 3,200 / 289 | 11.1 lb per hp |
- Step: Speed ratio
Calculation: 105 / 234
Result: 0.4487 - Step: Cubed
Calculation: 0.4487 x 0.4487 x 0.4487
Result: 0.09035 - Step: Engine horsepower
Calculation: 3,200 x 0.09035
Result: 289 hp - Step: Power at the wheels with a 15% loss
Calculation: 289 x 0.85
Result: 246 hp - Step: Weight for each horsepower
Calculation: 3,200 / 289
Result: 11.1 lb per hp
Chart: the estimate at different trap speeds
For a car of 3,200 lb. Each block is about 9.5 hp. A few mph makes a big change, because the speed is cubed. Your speed of 105 mph is green.
95 mph ███████████████████████ 214 hp 100 mph ██████████████████████████ 250 hp 105 mph ██████████████████████████████ 289 hp 110 mph ███████████████████████████████████ 332 hp 115 mph ████████████████████████████████████████ 380 hp
| Trap speed | Engine power | At the wheels (15% loss) |
|---|---|---|
| 95 mph | 214 hp | 182 hp |
| 100 mph | 250 hp | 212 hp |
| 105 mph | 289 hp | 246 hp |
| 110 mph | 332 hp | 283 hp |
| 115 mph | 380 hp | 323 hp |
Chart: the effect of the weight
All at 105 mph. A heavier car needs more power to reach the same speed. Each block is about 9 hp.
2500 lb █████████████████████████ 226 hp 3000 lb ██████████████████████████████ 271 hp 3500 lb ███████████████████████████████████ 316 hp 4000 lb ████████████████████████████████████████ 361 hp
The second estimate: from the elapsed time
If you enter the time of the quarter mile (the elapsed time, ET), the calculator gives another estimate:
horsepower = weight / (ET / 5.825) cubed
Example: 3,200 lb and 13.2 seconds give 3,200 / (13.2 / 5.825) cubed = 275 hp. The estimate from the trap speed is usually more reliable than the one from the time, because the time depends a lot on how well the car gets off the line (traction and reaction time). Each block below is about 12 hp.
15.0 s ████████████████ 187 hp 14.0 s ███████████████████ 230 hp 13.0 s ████████████████████████ 288 hp 12.0 s ███████████████████████████████ 366 hp 11.0 s ████████████████████████████████████████ 475 hp
Engine power and wheel power
The formula estimates the power of the engine, that is at the flywheel. The power that reaches the ground, at the wheels, is lower, because the gearbox, the driveshaft and the axles use some of it. A common loss is 10% to 15% for a manual gearbox and 15% to 20% for an automatic, and it is higher for an all-wheel drive. Enter your own loss in the calculator to see the power at the wheels.
How accurate is it?
- It is usually within about 10% for a car in good condition, but it is only an estimate.
- Weight: use the weight of the car with the driver, the fuel and anything else in it, as it crosses the finish line.
- Trap speed: use a speed from the timing slip, not from the speedometer.
- Things that change the result: the gearing, the tires, the weather (hot and humid air gives less power), the height above sea level and the wind.
- A dynamometer measures the power directly and is the better way if you need a precise number.
What this calculator does not include
- Motorcycles and very light cars, where the constant may not fit.
- Corrections for altitude, air temperature and pressure.
- Electric cars, which have a different power curve.
This tool gives an estimate for interest and comparison. It is not a measured value.