Hi. Wolfram Mathworld does such a nice job explaining this. Just look up torus and in 2 minutes you'll have it as well as the satisfaction of discovering it for yourself. Give it a try.
The circle x = a cost, y = a sin(t), 0 ≤ t ≤ 2π is revolved about the line x = b, 0 < a < b, thus generating a torus (doughnut). Find its surface area.
Area of the torus:
The formula I found for this is :
But what is R and r ? What do I need to do to find those?