Measure the height of the microwave. For example, your microwave is 11 inches high. Record this number on paper.
Measure your microwave's width. For example, your microwave is 18 inches wide. Record this number.
Measure your microwave's depth. For example, your microwave is 12 inches deep. Record this number.
Multiply all three of the numbers you have recorded. In our example, 11 times 18 times 12 equals 2,376.
Divide the product of the three dimensions by 1,728 to convert inches to feet. In our example, 2,376 divided by 1,728 equals 1.375, or approximately 1.4. This is the volume of the microwave in cubic feet.