How to Measure Medicine Cabinet Replacement | Get the Fit Right

Measure the width, height, and depth of the existing cabinet or rough opening to size a replacement medicine cabinet correctly.

Replacing a medicine cabinet seems simple until the new unit doesn’t fit the hole in your wall. Whether you’re swapping a recessed cabinet or mounting a new surface unit, a few precise measurements prevent a frustrating return trip. This guide covers exactly what to measure, how to account for rough openings, and the common mistakes that ruin a good fit.

What Measurements Do You Need?

You need three dimensions for any replacement: width, height, and depth. Width and height determine whether the cabinet fits the wall space; depth matters because a cabinet that sticks out too far can block light switches or door swing.

For a recessed cabinet, the rough opening in the wall is the critical number, not the visible front frame. Measure the opening’s height and width at its widest points — walls are rarely perfectly square, so take measurements at the top, middle, and bottom, then use the smallest figure. Depth is measured from the front edge of the wall studs to the back wall, accounting for any electrical wiring or plumbing that intrudes into the space.

For a surface-mount cabinet that sits on the wall, measure the available wall space instead, keeping clearance for the door to swing freely without hitting fixtures or wall obstructions.

How Does the Rough Opening Compare to the Cabinet Size?

The rough opening is always smaller than the cabinet itself. This gap allows room for shimming the cabinet level and plumb before final fastening.

For two or more cabinets installed side by side, add the cabinet widths together, then subtract 3/4 inch.

When measuring an existing recessed cabinet you plan to replace, you’re actually measuring the rough opening left behind — and that opening already reflects the old cabinet’s dimensions minus the clearance. Compare the new cabinet’s manufacturer dimensions against that opening, allowing the manufacturer’s specified clearance or rough-opening adjustment. Adding about 1/4 inch of wiggle room for test-fitting is common practice and gives you room to adjust.

Measuring for Recessed vs. Surface-Mount Installations

Recessed cabinets have overhanging flanges that cover the cut edge of the drywall — and measuring those flanges instead of the actual cabinet box is the most common sizing mistake. The flange is wider than the box, so a measurement taken across it tells you nothing about whether the box fits the opening. Always measure the actual cabinet box itself, not the mirror or the flanges.

Before cutting anything, check for studs, framing, and obstacles behind the wall. A stud running through the middle of your planned opening must be cut and reframed; electrical wires or plumbing pipes behind the wall may force you to relocate the cabinet entirely. Mark the intended opening on the wall before cutting, and test-fit the cabinet for level and plumb before final fastening. The door must also swing freely — a beautiful cabinet that blocks the light switch or hits the towel bar when opened is a failed installation.

For surface-mount cabinets, the math is simpler: the cabinet sits on the wall, so the wall space just needs to be larger than the cabinet’s width and height, with enough depth clearance for the door to open fully.

Common Sizing Mistakes to Avoid

Three errors cause most medicine cabinet fitting failures. First, measuring the overhanging flange instead of the cabinet box on a recessed unit. Second, assuming the rough opening matches the old cabinet’s nominal size — an old 15-inch cabinet often sits in an opening that has been modified, patched, or framed oddly over the years. Third, skipping the depth measurement entirely, which matters when the wall cavity is shallow or contains obstructions.

Once you have your rough opening dimensions, you can confidently compare them against any replacement’s specs. If you’re ready to shop, our roundup of the best 14 x 24 medicine cabinet options covers models that fit most standard openings.