Dimensions for kitchen appliances are typically standardized, though the industry standard depth of built-in microwaves varies from 12.5 inches to 22 inches. Here are some typical manufacturer depths for built-in microwaves: GE 15-1/4 inches; LG 15-3/8 inches; Frigidaire 15-1/32 inches and Whirlpool 15 -7/16 inches.
Standard depth for kitchen cabinet base units is 24 inches. The standard depth for cabinets above the base units is 12 inches although cabinet designers may vary dimensions slightly from this standard.
Regardless of the microwave depth, the cabinets installed above a built-in microwave should hold to the 12 inch industry standard for depth.