Well, hun, ya kinda gotta know how big the table is before you can get the right size fabric... so I might check up on that.
Here's the formula I'd use, once you have the exact measurements
Height of table to floor x 2
plus the diameter of the table
then convert to yards, and round up since you'll want some room for seams
You'll want to make sure the fabric you pick is really wide, too, so that the length will be the same all the way around.