Built-In Microwave Dimensions: The Trim Kit Rules
Built-in microwaves are sold at 24, 27 and 30 inches, but the cabinet opening is a different number: the trim kit frames an opening 24.5 to 30.5 inches wide and 17.5 to 19.5 inches tall, sized just over the microwave itself. Measure the opening, then buy microwave and trim kit as one decision.
How big is a built-in microwave?#
A built-in microwave is 24, 27 or 30 inches wide, 17 to 19 inches tall and 22 to 25 inches deep. Those are the case sizes, and none of them is the number your cabinet has to match: the trim kit frames an opening 24.5 to 30.5 inches wide and 17.5 to 19.5 inches tall, deliberately a little over the microwave so the flange covers the cut edge. Buying the microwave first and the trim kit afterwards is the usual way this install goes wrong, so the table below starts from the opening.
Built-in microwave dimensions by configuration#
| Configuration | Width (in) | Height (in) | Depth (in) | Best for |
|---|---|---|---|---|
| 24-inch microwave with trim kit | 24 | 17–19 | 22–25 | a flush cabinet face at the common width |
| 27-inch built-in microwave | 27 | 17–19 | 22–25 | a wider trim-kit opening in a tall cabinet |
| 30-inch microwave over a wall oven | 30 | 17–19 | 22–25 | stacking above a 30-inch wall oven |
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Your cabinet opening, and what it will accept#
Measure the finished opening in the cabinet face — width and height — and check what is above and behind it before choosing a class.
| What you measured | What it takes | What decides it |
|---|---|---|
| A finished opening inside the 24.5–30.5 in band | The class whose trim kit frames that exact opening | The trim kit, not the microwave, meets the cabinet face; the case behind it is 24, 27 or 30 in |
| A cabinet cutout height under 17.5 in | None of the three | Every class needs 17.5 to 19.5 in of cutout height for the case and its trim, against a microwave that is itself 17 to 19 in tall |
| A 30-inch wall oven already in the cabinet below | 30-inch microwave over a wall oven | The stack is sized as a pair — a narrower microwave above a 30-inch oven leaves a trim gap nothing closes cleanly |
| No outlet inside the cabinet | Nothing yet | A built-in microwave needs a grounded outlet inside the cabinet, usually in the space above the opening. That is wiring work before the trim kit goes on |
| Cabinet sides tight against the case | Any class — this is the design | It vents through the front of the trim kit and needs no rear or side air gap; a gap left "for airflow" only shows as a shadow line |
Three width classes are counted above, and each pairs with a trim kit that is ordered with it. The opening band is what the kits frame, taken from manufacturer installation sheets; it is deliberately a little over the case, and it is the number the cabinetmaker works to.
Installation clearances for a built-in microwave#
- Trim-kit opening width: 24.5–30.5 in in of cabinet opening a built-in microwave trim kit frames, sized just over the microwave.
- Cabinet cutout height: 17.5–19.5 in in of cabinet height a built-in microwave and its trim kit need.
- Rear venting: A built-in microwave vents through the front of its trim kit, so it needs no rear or side air gap in the cabinet.
- Outlet location: A built-in microwave needs a grounded outlet inside the cabinet, usually in the space above the opening.
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Will your built-in microwave fit? Clearances and door swing#
The number that actually governs a built-in microwave is the trim-kit opening, not the microwave itself. Your cabinet needs a finished opening 24.5 to 30.5 inches wide, sized just over the microwave so the trim kit can frame it, and 17.5 to 19.5 inches tall to swallow the unit and its kit. Depth of 22 to 25 inches has to clear inside the cabinet. Because a built-in microwave vents through the front of its trim kit, you do not leave a rear or side air gap the way you would for a freestanding oven — the cabinet can wrap it tightly. The one thing installers forget is power: the microwave needs a grounded outlet inside the cabinet, usually in the void above the opening, so confirm that outlet exists before you cut the hole.
Built-in microwave sizes by type: 24-inch microwave with trim kit, 27-inch built-in microwave, 30-inch microwave over a wall oven#
The 24-inch microwave with a trim kit is the common choice, giving a flush cabinet face at the width most tall cabinets are built around. The 27-inch built-in steps the trim-kit opening wider for a taller cabinet where you want more presence, without changing the 17-to-19-inch height or the depth. The 30-inch microwave over a wall oven is the stacking configuration: it matches the 30-inch width of the wall oven beneath it so the two read as one column. All three are the same height and depth, so the decision is purely about the cabinet width you are matching and whether the microwave stands alone or crowns a wall oven. Pick the width off the cabinet you already have, then let the trim kit close the gap.
Appliance Repair Technician · 8 years experience
Experienced technician with 8 years specializing in dishwasher repairs and European appliance brands including Bosch and Thermador.
