How to measure for a bathroom sink replacement starts with four dimensions: length, width, depth, and the vanity cutout.
Replacing a bathroom sink turns frustrating when the new basin arrives and doesn’t fit the cutout, the faucet holes don’t line up, or the drain misses the plumbing. The fix is measuring the right things before you buy. For a same-style swap, match the old sink’s length, width, and depth exactly. For a new style, the vanity cutout and old sink’s depth set your limits. Here’s what to measure and where to find the numbers that matter.
Length, Width, Depth: Which Measurement Goes Where
KOHLER’s buying guide defines length as side-to-side and width as front-to-back, and it distinguishes height from depth. That distinction causes most measuring mistakes. Height is the full exterior top-to-bottom measurement of the sink. Depth is the interior basin depth — how far water sits below the rim.
For a same-style replacement, match the old sink’s length, width, and depth. For a new style, measure the vanity’s existing cutout length and width plus the old sink’s depth, and make sure the replacement is no longer, wider, or deeper than those numbers.
A sink that fits but leaves no counter space makes the bathroom feel cramped.
Drop-In and Undermount Sinks: Measure the Cutout, Not Just the Rim
Drop-in and undermount sinks don’t just rest on the vanity — they nest into a hole cut in the countertop. That cutout is the gate. Measure its length and width at the widest points, not the sink’s outer rim. A sink can match the old one visually and still fail the cutout fit by half an inch.
This is where manufacturer spec sheets earn their keep. The KOHLER rough-in PDF for one model lists a basin area of 23-5/8 in. x 17-1/2 in., water depth of 3-1/4 in., drain hole of Ø 1-11/16 in., and faucet holes of Ø 1-3/8 in. A countertop that’s too narrow fails before the sink even arrives.
If you’re choosing a style for a new bathroom rather than replacing an old sink, check the new model’s spec sheet against your vanity before ordering.
Faucet Holes and Drain Opening: The Connection Details
The sink fits the counter, but the faucet and drain also need to fit the sink. Faucet hole spacing is not standard across manufacturers, so confirm the hole count and spacing against the new sink’s spec sheet before buying. KOHLER product sheets commonly list faucet hole diameter and spacing, typically Ø 1-1/4 in. to Ø 1-3/8 in.
Drain opening size matters just as much. KOHLER spec sheets commonly list a drain hole of Ø 1-11/16 in. (43 mm), Ø 1-5/8 in. (41 mm), or similar depending on model. The drain assembly and trap underneath must line up with the existing plumbing, so measure the opening and the distance from the wall or floor to the drain pipe.
For wall-mount sinks, the requirements shift entirely. A wall-mount sink won’t work if the plumbing doesn’t reach. When you’re ready to compare models and prices, our tested bathroom sink replacement roundup covers the options worth your money.
Measuring Checklist: Five Numbers Before You Buy
Run down this list with a tape measure and a notepad:
- Side-to-side length of the current sink.
- Front-to-back width of the current sink.
- Interior basin depth, measured to the overflow if one exists.
- Vanity cutout length and width for drop-in or undermount sinks.
- Faucet hole count, spacing, and drain hole diameter.
Small vanity-top sinks follow the same rules.
One honest note: if you’re swapping sinks in an existing vanity that was installed decades ago, expect surprises. Old cutouts rarely match modern spec sheets. Measure twice, compare against the manufacturer’s listed dimensions, and when the new sink’s numbers exceed the old cutout, it won’t fit without countertop work.
References & Sources
- Kohler. “Bathroom Sink Buying Guide and Rough-In Specification.” Defines length, width, height, and depth and lists cutout requirements.
