sink size calculator
Why sinks are always smaller than the cabinet
A kitchen sink never fills the full width of the base cabinet underneath it. Cabinet boxes need solid wood or particleboard on each side to hold the sink's mounting clips (or the countertop lip, for an undermount) — usually about 1.5" per side. That's why the rule of thumb is simple: take your cabinet's outer width and subtract 3" to get the largest sink that will actually fit and mount securely.
This applies whether you're replacing an old sink, building a new kitchen, or fitting a sink into an RV or tiny home cabinet. Measure the outside width of the cabinet box — not the countertop, not the opening inside the frame — and that's the number to plug into the calculator above.
Sink size by cabinet width
Here's the same math laid out for the most common base cabinet widths, if you'd rather skip typing:
| Cabinet width | Max sink width |
|---|---|
| 24" | 21" |
| 27" | 24" |
| 30" | 27" |
| 33" | 30" |
| 36" | 33" |
| 42" | 39" |
Sinks smaller than the max still work fine — the max is just the ceiling, not a target. If you're between two sizes, sizing down gives extra clearance and is always the safer call.
Common questions
What is my sink base cabinet size?
Open the cabinet doors under your sink and measure straight across the front, outside edge to outside edge. That number is your cabinet width. Once you have it, sizing the sink is easy — a 36" sink base needs a 33" sink.
What size sink fits a 30 inch cabinet?
A 30" base cabinet fits a sink up to 27" wide, which leaves about 1.5" of cabinet frame on each side for mounting.
Do I measure the cabinet or the countertop cutout?
Measure the cabinet's outer width, not the countertop. The countertop cutout is sized separately by the sink manufacturer's own template once you've chosen a sink, and your installer or fabricator will cut to that template.
Why can't the sink be the same width as the cabinet?
The cabinet needs solid material on each side to hold the sink's mounting clips or support the countertop's overhang. Without that margin, the sink has nothing to attach to and the edges of the countertop cutout would have no support underneath.
Is the 3 inch rule the same for undermount and drop-in sinks?
Yes, as a starting estimate — both styles need roughly the same margin of solid cabinet frame on each side. The exact number can shift slightly by sink brand and mounting hardware, which is why it's worth confirming against the sink's install template before cutting anything.
What if my cabinet is an odd size, like 28 or 31 inches?
The same math applies to any width — subtract 3" from your exact cabinet measurement. The calculator above handles any number, not just the common sizes.