Every child knows that helium balloons can fly high into the sky, but how much weight can it lift? Anyone can deduce the answers to these questions with a handful of facts and some basic math.
Helium's Lifting Power

Helium has a lifting force of slightly over one gram per liter. This lifting force comes from helium weighing less than air, resulting in an effect known as displacement. Because the surrounding air has been displaced by a substance with less weight per unit of volume, it is pushed away (upward, thanks to gravity) by the heavier substance.
A Balloon's Volume

The lifting force of a balloon depends on the size of the balloon. If we know the radius, we can approximate the volume by using the equation 4/3 x pi x the radius cubed. If we assume that an average balloon has a 30.48 cm diameter (one foot) and, therefore, a 15.24 cm radius, then the equation becomes 4/3 x 3.14 x 15.24 x 15.24 x 15.24, which equals 14,819 cm cubed, or 14.82 liters.
Calculating A Balloon's Lift

Assuming our balloon's volume is 14.82 liters, and if helium's lifting force is one gram per liter, that means it can lift slightly under 15 grams, or about 0.03 pounds.
References
 Photo Credit air balloon image by Ekaterina Shvigert from Fotolia.com