Saw Kerf and Cutting Waste in a Cut List
Why the saw kerf, trim and last-board drop change how much lumber you buy, and how to plan a cut list that does not come up short.
The kerf is real material you lose
A saw blade does not part wood along a line of zero width; it grinds away a strip the thickness of the blade and its set teeth. That strip is the kerf, and for a typical table saw it runs around 1/8 in, while thin kerf blades remove a little less and a bandsaw less still. Every cut in a run eats one kerf, so a board that looks like it should yield six pieces may only yield five once the losses stack up. Planning without the kerf is the classic way to come home one board short.
How the pieces per board is worked out
Start from the usable length, which is the stock length minus any trim you remove first. Within that space each piece needs its own length plus one following kerf, except the final piece needs no trailing cut. That gives pieces per board as floor((usable + kerf) / (piece length + kerf)), where the added kerf on top cancels the missing final one. Once you know pieces per board, the boards to buy is simply your quantity divided by that number and rounded up to the next whole board.
Trimming, defects and the usable length
Rough lumber rarely arrives with clean, square ends, and the last few inches often carry a check, a split or a snipe from the mill. Trimming a small amount off each board first gives you a true reference face and removes those defects before your good parts are cut. The trade-off is that trim comes straight off the usable length on every board, so a generous trim can quietly drop your pieces per board by one. Trim only what you need, and inspect boards so you can steer defects into the planned offcut.
Reading the offcut and buying spares
The offcut figure is the drop left after a full set of pieces and the kerfs between them, so it is the most you can rely on from a fully loaded board. Because the last board usually holds fewer pieces, its actual leftover is longer, which is handy stock for jig parts, spacers or test cuts. When your parts must all match in grain or color, buying one extra board is cheap insurance against a miscut. Keep the offcuts labeled by project and you will burn through them faster than you expect.