Get Accurate Plumbing Cutout Measurements When a Corner Cabinet Meets an Out-of-Square Wall

The Problem

Measuring straight off the inside corner to mark plumbing centerlines assumes the corner is a true 90 degrees. It almost never is — drywall mud buildup or ordinary framing tolerance leaves most corners a few sixteenths of an inch acute or obtuse. Drill the holes off that assumption and the cabinet goes in with the plumbing lined up wrong, because the cabinet itself won't sit flush against a corner that isn't square.

The Technique

1. Hold a straightedge — roughly the width of your cabinet — against the back wall to represent a true, straight reference line.
2. Slide a framing square along the front edge of that straightedge into the corner.
3. Check where the square actually touches: if it contacts the front edge but leaves a gap at the back (or vice versa), that gap is how far out of square the corner really is. In one example, a 3/16-inch gap at the back meant the whole cabinet would shift sideways by that amount once it was set against the corner.
4. Measure the width of your cabinet from the corner and mark that point — then shift the mark by the size of the gap you found, in the direction the out-of-square corner will push the cabinet. That mark is your corrected reference line.
5. Draw a plumb line down through the corrected mark, then measure your plumbing centerlines from that line instead of from the raw corner.
6. If the cabinet will sit against a filler strip, add the filler's width to your measurement before marking, since the filler pushes the cabinet body over by that much again.
7. Before drilling, map the whole cabinet's footprint on the wall — including checking where any vanity light fixture will land, since that can determine where the cabinet's centerline actually needs to be. Draw the cabinet's edge on the wall and measure back from that line for the final plumbing cutouts.
8. Cut the filler piece to close the gap only after the cabinet is set in place, not before — that way it's fit to the actual gap instead of a calculated one.