The kerf is charged twice and the skin is charged once
Slabbing a 36 inch block into 8 inch slabs looks like four slabs and change. Dress an inch off the top and bottom and there are 34 inches to work with. Four slabs at 8 inches is 32, three kerfs at 3/8 is 1-1/8, and the total is 33-1/8 — four slabs, with seven eighths of an inch left that will never be anything. A fifth slab would need another 8-3/8 and there is nowhere near that.
Then each slab gets laid out, and the kerf appears again between every piece. A 94 by 46 inch usable slab taking 36-1/2 by 24-1/2 cut sizes fits two across the length and one down the width, which is two pieces. Turn the piece and it fits three across and one down, which is three. Same slab, same saw, fifty percent more pieces, and the only thing that changed was which way round the layout was drawn.
Yield is a shape problem, not a waste problem
The instinct when a yield comes out at fifty percent is that something is being wasted. Usually nothing is. The block is 96 by 48 by 36, which is 96 cubic feet. Twelve finished pieces at 36 by 24 by 8 is 48 cubic feet. The other 48 went to the skin, to three slabbing kerfs, to the finish stock ground off every face, and above all to the strips left over when a 36-1/2 inch piece is laid on a 46 inch width and 9-1/2 inches has nowhere to go.
| Where it went | Cubic feet | Share of block |
|---|---|---|
| Finished pieces | 48.00 | 50.0% |
| Slabbing kerf | 2.82 | 2.9% |
| Skin, finish stock, offcut | 45.18 | 47.1% |
| Block bought | 96.00 | 100% |
The kerf is under three percent. Everything else is geometry, which is why the fix for a poor yield is a different piece size or a different block, not a thinner blade.
Plan the offcut before the layout is fixed
The strip left over on a slab is the cheapest stone in the shop, because it has already been paid for. A 9-1/2 inch strip down the length of a slab will not make another die, but it will make vase blocks, corner posts, sample plates or the small flush markers that get ordered singly and are annoying to buy block for. Shops that run well have a standing list of what the offcut becomes. Shops that do not have a yard full of it.
The calculator will not find those for you — it lays a single part out on a strict grid and stops. Treat its number as the floor rather than the answer, and look at the leftover dimensions it reports before you accept them.
What the arithmetic cannot see
It cannot see a seam, a knot of a different colour, an iron stain or a drill scar that goes deeper than the dressing allowance. Any of those turns a laid-out piece into scrap after the sawing is paid for, and none of them is predictable from three dimensions. A block bought sight unseen carries that risk and a block picked off the ground at the quarry carries less of it, which is most of the reason people still travel to look at stone.
Questions people ask
How many slabs come out of a block?
Take the block height, subtract the dressing allowance off both ends, and divide by the slab thickness plus one kerf, then round down — with the small refinement that the last slab does not need a kerf after it. A 36 inch block dressed an inch each end gives 34 inches; at 8 inch slabs with a 3/8 kerf that is four slabs and 7/8 of an inch left over. The leftover is where the temptation lies: it is never quite enough for another slab and it always looks like it should be.
How much does the saw kerf actually cost me?
Less than people expect at the slabbing stage and more than they expect at the layout stage. Slabbing a 96 by 48 block into four 8 inch slabs charges three kerfs across a large face, which comes to under three percent of the block. Cutting pieces out of a slab charges a kerf between every part in both directions, and on small parts that adds up quickly. The number that dominates both is neither: it is the strip of slab that is not wide enough for one more piece.
How much finish stock should I leave on a piece?
That is a shop figure and it belongs to your saw, your grinding sequence and how square the sawn faces come off. The field takes whatever number you work to. What the page does insist on is that the allowance is charged to the cut size, not to the finished size, because the pitch on the slab is cut size plus kerf and getting that wrong is how a layout that fitted on paper comes up one piece short.
Why does turning the piece change the count so much?
Because the leftover strip changes. On a 94 by 46 usable slab, a 36-1/2 inch cut length fits twice across 94 with 21 inches left, and a 24-1/2 inch cut width fits once down 46 with 21 left. Turn it and the 24-1/2 fits three times across 94, leaving 20. The block did not change and neither did the saw; the number of times a dimension divides into another dimension did. Whenever the grain or colour run does not force the direction, the layout should be drawn both ways before anything is cut.
Does this include polishing and finishing time?
No. It accounts for the stock removed by finishing as volume, because that stock has to be bought, but the time and the consumables are a separate estimate driven by how many square feet of each finish the job carries rather than by how many cubic feet came out of the block. Work the finished faces out on the monument finish area calculator and price the finishing off that.