Two strips and a hole
Draw the section through a sink and the top stops being a countertop. In front of the opening there is a strip of stone as wide as the setback. Behind it there is another strip as wide as whatever is left. Between them there is nothing. Across the width of the sink, the entire top is those two strips, joined only at the two ends of the opening.
On a 25.5 inch counter with a 17 inch opening set back 2.5 inches, the two strips are 2.5 inches and 6 inches. That is the whole structure across a thirty inch span, and the 2.5 inch strip is the one that lives at the front edge where people lean on it, put a heavy pan on it, and where the counter gets hit going round a corner.
The faucet holes make it worse and nobody notices
The back rail looks like the comfortable one. It is not, once it is drilled. Three holes at 1-3/8 inch diameter on centres 2-1/4 inches from the back edge take out a band from 1-9/16 to 2-15/16 inches measured from the wall. What is left is a 1-9/16 inch strip behind the holes and a 3-1/16 inch strip in front of them.
So the narrowest stone in the top is not the 2-1/2 inch front rail everybody worries about. It is the 1-9/16 inch strip behind the faucet, which nobody drew, because the faucet position was set by the faucet and the hole positions were set by the template. The calculator prints every strip, in order, and marks the thin one, precisely because the thin one is usually not the one being discussed.
| Strip | Set by | Who decided it |
|---|---|---|
| Front rail | Setback from the front edge | Whoever placed the sink on the drawing |
| Back rail behind the holes | Hole centre distance from the wall | The faucet, mostly |
| Back rail in front of the holes | Depth minus everything else | Nobody — it is what is left |
Where the setback number comes from
The setback is the only one of these you get to choose freely, and it is chosen early, usually without much thought. Push the sink back and the front rail grows while the back rail shrinks, until the faucet has nowhere to go. Pull it forward and the front rail becomes a sliver at the exact place the top gets abused. The arithmetic is a straight trade because the three numbers have to add to the counter depth, which is what the output line at the end of the first block is checking.
The practical constraint on the back is the faucet, the backsplash and the wall not being flat. The practical constraint on the front is that a narrow front rail is fragile in handling and unattractive in use. There is no correct answer here that this page can hand you, because it moves with the material, the thickness, whether the fabricator rods the section, and what the sink actually is. It is a question for the shop, and the useful thing to bring to the shop is the number, not a feeling.
The piece that comes out
A 30 by 17 inch offcut in 1-1/4 inch material at 165 pounds a cubic foot is about 61 pounds of stone. It is already paid for, it is not going back to the supplier, and it is the reason people occasionally end up with a chopping board, a hearth pad or a small shelf they did not order. Ask for it before it goes in the skip if you want it; most shops will hand it over and it saves them the disposal.
The offcut also tells you something about the hole. Sixty-one pounds is what came out of the middle of a piece that then has to be lifted, turned on edge and carried in. The piece is lighter after the cut and much more fragile, and the two facts are related — the material that was removed was the material that made it a slab rather than a frame.
What this page will not tell you
It does not say whether a rail width is enough. There is no general answer to that: it depends on the material, the thickness, the length of the opening, whether the section is reinforced, how the piece is handled, and what the fabricator will warrant. This page reports the geometry and stops there. If a rail comes out narrower than you expected, the useful next step is a phone call with the number in front of you, not a search for a rule of thumb.
For the rest of the top, the countertop square footage calculator handles area, edge and cutout counts across the whole kitchen, and the seam placement calculator keeps the seam away from the opening this page is measuring. Underneath, the cabinet run and filler calculator works out the box layout the sink base has to fit into.
Questions people ask
What is the standard setback for a sink cutout?
There is not one, and the range in real kitchens is wide. What sets it in practice is the sink, the faucet and the counter depth: the opening plus the setback plus the back rail have to add to the depth, so choosing any two fixes the third. Two and a half inches from a finished front edge is a figure a lot of people work near on a 25 to 26 inch counter with a 17 to 18 inch opening, but that is an observation about arithmetic rather than a specification. Your fabricator will have a view for the material you are buying, and that view is the one that counts.
Why is the back rail narrower than it looks?
Because the faucet holes go through it. Enter the hole diameter and the centre distance from the back edge and the output splits the back rail into the strip behind the drilling and the strip in front of it. On the defaults here a 6 inch back rail becomes a 1-9/16 inch strip and a 3-1/16 inch strip, neither of which is 6 inches. That drilling line is decided by the faucet base and the template, usually after the top has already been designed, which is why it surprises people.
Does an undermount sink need a bigger cutout than a drop-in?
It needs a different one, and the difference is in the finish rather than the size. A drop-in sits in a rough opening covered by its own rim, so the cut edge never shows. An undermount opening is the visible edge of the counter, so it is cut to a template, polished all the way round and usually eased, and the tolerance on it is far tighter. Both consume the same square footage of slab, both leave the same rails, and both are charged as a cutout. What differs is labour and how forgiving the cut is.
Can the calculator tell me if my rail is too narrow?
No, and it is built not to. Whether a given strip of a given material at a given thickness across a given opening is acceptable is a materials and workmanship question, and the honest answer to it comes from the fabricator and the material manufacturer, not from three dimensions. What the page does is make sure you know which strip is the narrow one before that conversation, because the strip people bring to the conversation is usually the front rail and the narrow one is often behind the faucet.
How much does the offcut from a sink hole weigh?
For a 30 by 17 inch opening in 1-1/4 inch material at 165 pounds a cubic foot, about 61 pounds. Change the thickness to 3/4 inch and it drops to around 37; take the density to 180 and it goes back up. It is a genuinely awkward object — heavy, thin, with a polished face and unfinished edges — and it is why a shop will cut it up rather than hand it over whole unless you ask. Enter your own density; it varies enough by material and colour to move the answer noticeably.