Stretch a tape measure across the width of the highest shelf in your shelving system. Record the measurement in inches. Stretch the tape from the front of the shelf to the back and record this dimension. Stretch the tape from the surface of the shelf up to the ceiling and record the measurement.
Multiply these three measurements together with a calculator. Divide the result by 1,728 to arrive at the shelf's capacity in cubic feet.
Repeat the previous two steps for each of the shelves below the top shelf, measuring the width, depth and height of the space available.
Add all the resulting figures together to arrive at the total capacity of the shelves in cubic feet.