Kerf is the width of the slot a saw leaves behind. It comes from the teeth, which are wider than the blade body so the plate doesn't rub in the cut, plus any extra width from runout or a slightly bent blade. Whatever the number is, that much wood turns to dust on every pass.
Why kerf matters in a cut list
A drawing shows parts touching each other. Real parts have a cut between them. Rip a 48-inch-wide sheet into four 12-inch strips and the numbers only work on paper: three cuts at 1/8 inch remove 3/8 inch, so the last strip comes out 11 5/8 inches wide. Ten rips at 1/8 inch remove 1 1/4 inches, which is enough to lose a part.
The same thing happens along a board. You cannot get four 24-inch pieces from a 96-inch 2×4: after three cuts you have three good pieces and one that is 23 5/8 inches long. Board ends are often not square, either, so you usually lose a little more to trimming.
The loss depends on how many cuts you make, not on how big the parts are, so projects with many small pieces (drawer boxes, face frames, shop jigs) are the most sensitive to it.
| Number of cuts | 1/16 in kerf | 3/32 in kerf | 1/8 in kerf |
|---|---|---|---|
| 3 | 3/16 in | 9/32 in | 3/8 in |
| 5 | 5/16 in | 15/32 in | 5/8 in |
| 10 | 5/8 in | 15/16 in | 1 1/4 in |
| 20 | 1 1/4 in | 1 7/8 in | 2 1/2 in |
Typical kerf by tool
These are typical ranges, not specifications. Kerf varies with the blade maker, diameter and tooth design, so use the table as a starting point and measure your own blade.
| Tool and blade | Typical kerf (in) | Typical kerf (mm) |
|---|---|---|
| 10 in table saw, full-kerf blade | about 1/8 (0.125) | about 3.2 |
| 10 in table saw, thin-kerf blade | about 3/32 (0.094) | about 2.4 |
| 7 1/4 in circular saw | 1/16 to 3/32 (0.06 to 0.10) | 1.6 to 2.4 |
| Track saw | 0.087 to 0.098 | 2.2 to 2.5 |
| Miter saw, 10 or 12 in | 3/32 to 1/8 (0.09 to 0.13) | 2.4 to 3.2 |
| Bandsaw | 0.025 to 0.04 | 0.6 to 1.0 |
| Jigsaw | about 1/16 (0.06) | about 1.5 |
| CNC router | bit diameter, e.g. 1/4 (0.25) | e.g. 6.35 or 6 |
A bandsaw's kerf depends on the blade thickness and the amount of set in the teeth, which is why it covers a range. A jigsaw blade flexes and wanders, so treat its kerf as a rough figure and leave extra room around jigsaw cuts. A CNC router is the odd one out: the kerf is the cutter's diameter, so parts nested with a 1/4-inch bit need 1/4 inch between them. If your CAM software adds its own spacing, enter that figure instead.
How to measure your own kerf
The kerf printed on a blade's packaging describes the teeth. The slot in the wood can be a few thousandths wider if the arbor has runout, the plate is not perfectly flat or the teeth carry pitch buildup. Two ways to measure it:
- Direct measurement. Cut partway into a flat piece of scrap, then measure the slot with the inside jaws of a pair of calipers, or find the feeler gauge that slides in without forcing. Check near the top and the bottom of the slot.
- The averaging method. Measure a straight board precisely, crosscut it into several pieces, then measure the pieces and add them up. The difference divided by the number of cuts is your average kerf. A 24-inch board cut into five pieces takes four cuts; if the pieces total 23 9/16 inches, the cuts removed 7/16 inch, or about 0.109 inch (2.8 mm) each.
Measure each saw separately and repeat the test after changing blades. If some parts in a layout will be cut with a track saw and others on a table saw, use the larger of the two kerfs so every cut has room.
Which side of the line to cut on
The kerf has to come out of the waste, never out of the part. Mark the part, then put the blade on the waste side so the cut just leaves the pencil line on the piece you keep; woodworkers call this leaving the line. A blade centered on the line takes half a kerf from the part, about 1/16 inch with a full-kerf blade, and parts marked end to end along a board come up short one after another.
A table saw handles this for you: the part between the blade and the fence is the measured piece, and the kerf comes out of whatever is on the other side. In a series of rips, each pass shrinks the remaining panel by the strip width plus one kerf, which is why the last strip is the one that comes out narrow. Rip the widths that matter most first.
With a track saw, lay the track on the part you are keeping. The edge of the splinter strip marks the cut, and the kerf falls on the waste side. With a circular saw and a clamped straightedge, measure the distance from the blade to the edge of the saw's base and set the guide on the keeper side by that amount.
What happens if you set kerf to 0
A kerf of zero tells the optimizer that parts can share an edge. The layout looks tighter than reality and can promise parts that won't be there. The figure shows eight 47 × 12-inch shelves on a 4×8 sheet. At zero kerf, four rows of 12 inches fill the 48-inch width exactly and all eight shelves fit. At 1/8 inch, four rows need 48 3/8 inches, so only six shelves fit and the other two go on a second sheet.
Zero kerf is only reasonable for rough material estimates, or if you have already added the kerf to each part size yourself. Otherwise enter your measured kerf, rounded up to the nearest 1/32 inch or 0.5 mm. Rounding up costs a little yield; rounding down can cost a part. If you plan to rough-cut pieces oversize and clean them up later, add that allowance to the kerf so the layout reserves the material.
Kerf and edge trim together
Kerf is one of two allowances that take material off a sheet. The other is edge trim: the strip removed from a damaged or out-of-square factory edge. An optimizer handles them differently. Kerf is reserved between neighboring parts, while trim comes off the outside of the sheet before any parts are placed. The guide to breaking down plywood covers when to trim and how much.
Once you know your number, enter it in the sheet cut list optimizer for panels or the lumber cut list optimizer for boards, where every crosscut takes the same toll along the length. If you are still drafting the parts, start with how to make a cut list.