STANDARD AND LARGE MICROWAVE DIMENSIONS Widths can range from about 21 to 25 inches. If you're installing in cabinets, a trim kit will usually bring the width to 27 or 30 inches. Height is usually around 12 to 15 inches. Closed-door depths range from roughly 16 to 20 inches.
Microwave size Height in Inches Width in Inches
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Small 10-14 15-19
Medium 12-15 20-24
Large 15-18 26-30
Is there a standard size for over the range microwaves?
If you look at all of the available variants of microwave ovens on the market in terms of width, height and depth, you'll see that there really are no standard sizes, but microwaves designed for over-the-range installation all fit into a 30-in. wide space.
Can you put a 30 inch microwave over a 36 inch cooktop?
It is a REALLY bad idea to have an OTR MW over a 36" cooktop. ... Even a 30" MW over a 30" cooktop doesn't do much because there is very little capture area other than a couple of squares toward the back.