Sink cutout dimensions: how to calculate them correctly

Most worktop failures we hear about are not sink defects. They are cutout errors: a hole 5 mm too large, a corner left square where the bowl has a radius, or a tap hole drilled before anyone checked which way the spout swings. None of these are recoverable without a new piece of stone.

The three mounting types cut differently

The cutout is not derived from the sink’s overall size. It is derived from the mounting method, and getting that wrong is the first mistake.

A topmount sink covers the cut edge with its flange, so the hole is smaller than the sink. An undermount is fixed from below, so the hole follows the inside of the bowl and the stone edge is visible. A flush sink sits level with the worktop, which means the cut has to be exact in both dimensions and in depth.

Worked example: a 760 mm undermount bowl

Take a bowl with an internal width of 700 mm and an external width of 720 mm, wall thickness 1.5 mm, corner radius 10 mm.

• Cutout width: 700 mm, or 702 mm if you want a 1 mm shadow line on each side.
• Corner radius: 10 mm minimum, matched to the bowl radius. A square corner in stone concentrates stress and will crack.
• Cutout depth from the front edge: set by the cabinet, not the sink. Confirm the bowl is centred on the cabinet opening, not on the worktop.
• Tap hole: 35 mm for a standard mixer, positioned so the spout clears the back edge of the bowl by at least 40 mm.

The number that surprises people is the corner radius. A router bit cannot cut a sharper internal corner than its own radius, so if the bowl has a 10 mm radius and the fabricator only has a 12 mm bit, the cutout will not accept the sink without hand finishing. Ask for the bit size before the stone is cut.

Tolerance by worktop material

Stone is dimensionally stable but brittle; timber moves. That changes the allowance.

• Quartz and sintered stone — ±0.5 mm is achievable and should be specified. No expansion gap needed.
• Granite — ±1 mm, and check for fissures near the cut line before cutting.
• Solid surface — ±0.5 mm, and the cut edge must be sealed and sanded.
• Timber and laminate — ±2 mm, plus a 2 to 3 mm expansion gap on the long dimension, sealed against water ingress.

The four mistakes that cost a worktop

Using the packaging size as the cutout. Carton dimensions include foam and are 20 to 40 mm larger than the sink. Always work from the technical drawing.

Forgetting the radius. A square cutout under a radiused bowl leaves the corners unsupported and the sink rocks. It also cracks stone at the corner, usually within weeks.

Drilling the tap hole first. Tap position depends on the bowl position, which depends on the cutout. Cutting the tap hole before the sink is set in place is how a spout ends up over the drainboard instead of the bowl.

Ignoring the cabinet below. A deep bowl with a rear drain can foul a drawer box. Check the internal cabinet clearance before approving the cutout, not after the worktop is on.

What to ask a supplier for

Every sink we ship can be accompanied by a dimensioned drawing in DXF or PDF, with the cutout, the corner radius, the tap hole positions and the flange thickness all stated. If a supplier cannot provide that, the cutout is being guessed on site, and the risk sits with the fabricator.

For custom sizes, the drawing also serves as the approval document: it is signed before production, and it is the reference if a dispute arises later. Our custom sizing service works this way, and the mounting comparison covers the structural differences between the three options in more detail.

One last point on sequencing. Confirm the cutout drawing against the actual cabinet on site before the stone is templated. Templates made from a drawing rather than a cabinet are the single most common cause of a sink that fits the hole but not the kitchen.