Question: are all microwaves the same size?
Answer: All 30" over-the-range microwaves are approximately the same width, and are designed to fit into a 30" wide cabinet opening. ... If you are replacing a 36" over-the-range microwave, many current 30" over-the-range microwaves offer an optional 36" filler kit to allow it to be installed in a 36" cut-out opening.