Sometimes students have problems remembering formulas for area or volume. One easy way to know the volume of a cylinder is to think of a cylinder as a bunch of very thin circles stacked on top of each other. Think of a CD and stacking them on top of each other to a certain height. Assuming there was no hole in the middle of the CD, it's basically a cylinder, but if you think of it as just a circle, you can get the area of the circle and multiply it by the height of the stack. That is the volume of a cylinder.

We know the area of a circle is Pi times radius squared or Pi(r^2). Therefore the volume of a cylinder is Pi times radius squared times height or Pi(r^2)h.