Open the door of your microwave and measure the length, width and height in inches of the interior cavity.
Use your calculator to multiply the length by the width by the height to get the total in cubic inches.
Divide the cubic inches by 1,728, which is the volume of a cubic foot. The resulting number will be your microwave's measurement in cubic feet. If you do not want to do the math, you can use a conversion calculator such as the one available on the Metric Conversions website (see Resources).