Measure the length of your storage area by extending the tape measure and measuring across the longest side of the storage space.
Repeat your measurement for the width of the storage space. This will be the shorter side.
Measure the height of the storage area. You may need to stand on something to fully extend the tape measure up the wall.
Multiply the measurements for length, width and height. As an example, if you measured a storage space to be 8 feet by 6 feet by 10 feet, your calculated storage space is 480 cubic feet.
If the area is triangular, such as the corner of a room or attic, take this previous measurement and divide by 2. To continue the example, 480 cubic feet / 2 would equal 240 cubic feet.