Measure the height of your old microwave oven with measuring tape. The height extends from the microwave oven's exterior bottom to its exterior top. Write the measurement on paper.
Find the microwave oven's depth by measuring from the exterior of the appliance's front to the exterior of its back. Write down the measurement.
Measure the width of the microwave oven, and note the measurement on paper. The width encompasses the space from the exterior of one side of the microwave oven to the exterior of its other side.