Measure the interior length, width and height in feet of the refrigerator inside the cooling chamber. Only measure parts of the refrigerator inside when it the door is closed.
Multiply the length times the width times the height. For instance, if the inside measured 2 feet by 3 feet by 2 feet, the volume would be: 2 x 3 x 2 = 12 cubic feet.
Find the average usable space inside your refrigerator by multiplying the total cubic feet by 65 percent if you have a side-by-side, or 78 percent if the freezer is located on the top or bottom. For the example, if a 12-cubic-foot refrigerator is a side-by-side, calculate: 12 x 65 percent = 7.8 cubic feet. If the freezer on top, calculate: 12 x 78 percent = 9.36 cubic feet.