What is saw kerf, and why it ruins your last part
Kerf is the slot a saw blade cuts — the material that turns into sawdust with every pass. It sounds like trivia until you do the arithmetic on a full sheet: cut a 48″ panel into four "12-inch" strips and you'll get three at 12″ and one at about 11⅝″, because three cuts each ate an eighth of an inch.
Every accurate cut list accounts for kerf. Here's how much to allow, and how.
How wide is a saw kerf?
Kerf depends on the blade, not the saw. Typical widths:
| Blade type | Typical kerf | In millimeters |
|---|---|---|
| Standard (full-kerf) table saw blade | 1/8″ (0.125″) | ~3.2 mm |
| Thin-kerf table saw blade | 3/32″ (0.094″) | ~2.4 mm |
| Track saw blade | ~0.087–0.094″ | 2.2–2.4 mm |
| Circular saw blade (framing) | ~1/8″ or more | 3.2+ mm |
| Bandsaw blade | ~1/32–1/16″ | 0.8–1.6 mm |
| CNC router bit (¼″) | 1/4″ (0.25″) | 6.35 mm |
The measurement that matters is the width of the finished slot, which is set by the blade's tooth width (the "set" or carbide overhang), not the plate thickness printed on the packaging. When in doubt, make a test cut in scrap and measure the slot.
Why kerf compounds
One kerf is an eighth of an inch — invisible. But kerfs accumulate with every cut in a row of parts:
- 4 parts ripped from one panel → 3 internal cuts → ⅜″ consumed
- 8 drawer parts crosscut from one board → 7 cuts → ⅞″ consumed
That's why the failure always shows up on the last part: everything upstream measured true, and the shortfall quietly collected at the end of the board. If a layout was planned "edge to edge" with no kerf allowance, the final part is short by one kerf per cut before it.
Rule of thumb: a layout that looks like it just barely fits on paper does not fit in the shop. If parts sum to within a few eighths of the stock dimension, count the cuts.
How to allow for kerf in a cut list
When planning by hand, insert one kerf width between every pair of adjacent parts in the layout — not after the last one. The check for any row of parts is:
sum of part widths + (number of cuts × kerf) ≤ stock width
In practice this is tedious to track across a whole sheet with parts of mixed sizes, which is why cut list optimizers treat kerf as a first-class input: you set your blade's kerf once, and every layout and every cut sequence reserves it automatically. Cutlist defaults to ⅛″ and lets you match it to your blade, in inches or millimeters.
Kerf vs. other allowances
Kerf isn't the only material that disappears between the lumberyard and the finished part:
- Jointing and planing allowance — rough lumber loses thickness and width getting flat and square.
- Trim cuts — factory edges of plywood are rarely straight or square enough to reference; most builders skim them off, losing ¼″–½″ per edge.
- Nominal vs. actual stock sizes — the sheet itself may not measure what its name says. See plywood sheet sizes.
A cut list that survives contact with the shop budgets for all three, plus kerf.
Let the software carry the eighths
Cutlist reserves your blade's kerf in every sheet and board layout, so the last part comes out the same size as the first.
Try the cutlist calculator — free