How to Measure for Undermount Kitchen Sink?

Measure the cutout opening, the sink’s outer rim-to-rim dimensions, and the vertical clearance under the counter for a proper undermount fit.

Getting the numbers right for an undermount kitchen sink means measuring three distinct things: the existing countertop opening, the sink’s full outer dimensions, and the vertical space beneath the counter. Each one answers a different question, and skipping any of them leads to a sink that won’t sit flush, a cutout that’s too small, or a bowl that collides with the disposal.

The most common mistake is measuring the interior bowl instead of the outer rim. That error produces a cutout too small for the sink to drop into. Home Depot’s installation guidance stresses measuring the sink’s overall length and width from rim to rim, not the basin’s inside edges. For a replacement, the existing cutout is your starting point; for a new install, the manufacturer’s template and the cabinet interior set the limits.

Start With the Cabinet Interior

The base cabinet determines the maximum sink size before you touch the countertop. Measure the interior width from wall to wall, then subtract a small allowance on each side for the mounting clips and the sink’s rim flange. The sink’s external width must fit inside that reduced opening.

Depth matters just as much. Measure from the countertop’s underside down to the cabinet floor, then account for the bowl depth plus the drain assembly. A 9-inch-deep bowl needs roughly 9 to 12 inches of vertical room once you add the tailpiece and trap. If a garbage disposal is planned, include its body length in that clearance check—it’s a common fit issue that surfaces only after the sink is already mounted.

Measure the Sink and the Cutout Correctly

Length: left-to-right across the outer rim. Width: front-to-back across the outer rim. Depth: from the top rim to the basin’s lowest point. These three numbers come straight from the sink’s spec sheet and must match what your cabinet can physically hold.

For the cutout, measure edge to edge. If the opening isn’t a perfect rectangle—and many aren’t—measure each curved or irregular section separately and record the corner radii. Rounded corners are easy to miss, and a square sink dropped into a rounded-corner cutout leaves visible gaps that no caulk will hide. Check multiple points along the length and width to catch any out-of-square or uneven sections.

Home Depot’s installation guide for undermount sinks offers a useful reference point: one example sink spec lists external dimensions of 30″L × 18″W × 9″D with an interior of 28″L × 16″W × 9″D. The several-inch gap between outer and inner measurements is exactly why measuring the rim matters more than the basin.

Check Plumbing, Faucet, and Backsplash Clearance

Behind the sink sits a row of potential conflicts. The faucet needs its own space, and the backsplash limits how far back the sink can sit. Rough drain outlets must line up with the sink’s drain hole, or you’ll be cutting new plumbing runs mid-installation. Leave room for the mounting clips and any accessories like a soap dispenser or side sprayer.

Undermount sinks hang slightly below the countertop edge, which means the countertop itself must support the sink’s full weight plus whatever sits in the basin. For stone or solid-surface tops, the fit is especially critical—the cutout edge carries the load and needs a clean, sealed finish. Consult the sink’s spec sheet and template before purchase; hold the template up against the cabinet and counter opening to verify the fit before any cutting begins.

Once the measurements are in hand, comparing specific models becomes straightforward. If a 27-inch option fits your cabinet and cutout, our tested roundup of the best 27-inch undermount kitchen sinks breaks down bowl depth, material, and mounting hardware side by side.

When the sink arrives, place the actual unit or its template on the counter before committing to the cut. This final check catches measurement drift and confirms the clip holes, faucet holes, and drain positions all line up as planned.