Vikki, that's exactly how it works. If you know the torque at a given rpm, you use the formula to determine the horsepower, and vice versa.

Before we get into the age old battle about which is more important to a street car, torque or hp, remember that they are directly related. When you say you have 500 ft.lbs of torque at 4000 rpm, you are also saying that you have 381 hp at 4000 rpm.

Most people would say that torque is what accelerates a car. Guess what, so does horsepower! Horsepower is just a useful number that is converted from torque. I think I did this before but here's an example (for those who are still awake): two engines make the same horsepower and are installed in two identical cars. If we freeze-frame the two cars, when both cars are at the same speed and both engines are producing the same horsepower, they will accelerate at the same rate. Makes sense right? Now for the punch line: it doesn't matter what the gear ratio or the size of the engine is - or how much torque it is making.

When you have this figured out, you'll understand why horsepower numbers are more useful than torque numbers, and why most power ratings are in horsepower.