Shipping suppliers sell packing peanuts and styrofoam balls by volume, rather than by weight. In the US, businesses and individuals buy packaging filler by the cubic foot. The packaging volume is simply the difference between the volume of the shipping container the volume of the object you are shipping. If you know how to calculate packaging volume, you can purchase the right amount of packing material.
Things You'll Need
 Measuring tape
 Calculator

Measure the width, length and height of the shipping box in inches.

Multiply the width, length and height of the box to calculate its volume in cubic inches. For example, if the box is 20 inches wide, 24 inches long and 16 inches high, then the volume is (20)(24)(16) = 7,680 cubic inches.

Measure the width, length and height of the object that you are shipping, in inches. If the object is not a perfect rectangle, take the average width and length. For example, if the object is 10 inches at its widest point and 6 inches at its narrowest point, use 8 inches for the width. If you are wrapping the object in bubble wrap, take these measurements after you wrap it.

Multiply the width, length and height to find the volume of the object you are shipping. For example, if the object is about 8 inches wide, 15 inches long and 10 inches high, then the volume is about (8)(15)(10) = 1,200 cubic inches.

Subtract the volume of the object from the volume of the container to find the packaging volume. For example, if the box is 7,680 cubic inches and the object is 1,200 cubic inches, then the packaging volume is 7,680  1,200 = 6,480 cubic inches.

Convert cubic inches to cubic feet by dividing by 1,728. For example, 6,480 cubic inches is equal to 3.75 cubic feet. This is the volume of packing peanuts or other packaging material that you should buy.
References
 Photo Credit background styrofoam peanut image by robert mobley from Fotolia.com