Here's some intersting info on helium:
1. Determine the volume of the balloon.
The volume of a sphere is 4/3 * pi * r3, where r is the radius of the balloon. So first determine the radius of the sphere (the radius is half the diameter). Cube the radius (multiply it by itself twice: r*r*r), multiply by 4/3 and then multiply by Pi. If you are measuring your balloon in feet, that gives you the volume of the balloon in cubic feet.
2. One cubic foot of helium will lift about 28.2 grams, so multiply the volume of the balloon by 28.2.
3. Divide by 448 -- the number of grams in a pound -- to determine the number of pounds it can lift
Obviously, the table isn't a sphere, but if you can calculate the volume of helium it can take, you should be able to determine how much weight it will lift.