30" cabinet.

This microwave oven will be hung over a range or cooktop, right? You should also allow at least 30" between the top of the range/cooktop and the bottom of the cabinet to which you will attach the oven. The oven will also attach to the wall behind it.

Why are you putting the range/cooktop/microwave next to a refrigerator? You won't have any counter space next to the range/cooktop -- you should have at least 18" of counter space on each side -- and you'll be putting a heating appliance right next to a cooling appliance if you do that.

@wdccruise - the OP says nothing at all about an OTR MW, but is specific that she is looking for an upper cabinet to place the MW.

To the OP - look for @darbuka's pictures of her MW on a shelf in her kitchen, and you can see a good setup for what you are talking about.

We just did this. I measured the microwave, added the clearances needed for each dimension (from the manual), and that was the minimum interior dimension of the cabinet. Then use the cabinet specs to determine the minimum cabinet size to obtain those inside dimensions. For my kitchen I didn’t want that microwave cabinet to be deeper than the surrounding cabinets, so I actually ended up buying the microwave we intended to use and measuring the depth needed so that the bottom feet would be secure on the shelf, but the front (basically just the door) will be protruding beyond the cabinet.
I overestimated to give a bit of wiggle room for the possibility of needing to replace the microwave with a different model some day, but it sounds like your current microwave is already large.

Cruise - Our undercabinet MW is adjacent to our fridge and there is no issue; there's an air gap between them. And, we have counter space below, next to the range.

Microwave ovens are far less expensive than cabinets. The only MW ovens that are 30" wide are the over-the-range types. Here you see a standard OTR microwave and a low profile version.

See images below for portable microwaves that will fit on a shelf in a kitchen. These cabinets are typically 24" wide and vary in depth.

Back in the 90s prior to the "over the counter" models, I installed an "over the range" microwave as a built in over a countertop. We just didn't hook up any ductwork. It worked at the time. But now Sharp has an Over the COUNTER model.

This is a 24" over the counter model with 14" deep upper cabinets.

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@M Miller: "OP says nothing at all about an OTR MW"

@User: "The only MW’s with thise dimensions are OTRs."

Yup.

Can you post the model number of your microwave?? This way we can all see the dimensions you are working with.

If it is a regular microwave, are you using a trim kit? If so then the cabinet must be much deeper than a regular cabinet.

We just need a few more details to help you.