The definition
Kerf is the slot a saw blade leaves behind, or more precisely the width of material removed by the cut. It is turned into sawdust or chips. Every cut has a kerf, whether you use a hand saw, a circular saw, a band saw or a table saw. A thin hand saw leaves a narrow slot, and a thick blade on a power saw leaves a wider one.
Why the kerf is wider than the blade plate
A blade’s kerf is usually a little wider than the metal plate itself. The teeth are bent outward (called set) or built wider than the plate, so the blade does not bind in the cut. Carbide-tipped blades have teeth that stick out past the body of the blade. The result is a kerf that is the width of the tooth path, not the thinner steel behind it.
Typical kerf widths vary by blade type, but you can think in rough terms: thin-kerf circular saw and table saw blades are often around one-eighth of an inch or a little less, and standard blades are somewhat wider. Hand saws and fine-toothed backsaws often cut narrower slots. Exact widths vary by maker and model and are often printed on the blade or its packaging, so measure or check rather than assuming.
How kerf affects your measurements
When you cut a board, the kerf comes out of the material, not out of either piece. The finished lengths of the pieces add up to less than the board you started with.
A worked example
Suppose you want to cut three equal pieces from a board that is 36 inches long, and your blade has a kerf of 1/8 inch (0.125 inch). This is a hypothetical example.
- Three pieces need two cuts.
- Material lost to kerf: 2 × 0.125 = 0.25 inch.
- Material left for the pieces: 36 − 0.25 = 35.75 inches.
- Each piece: 35.75 ÷ 3 ≈ 11.92 inches, about 11 15/16 inches.
If you had planned on three pieces of exactly 12 inches, you would have needed 36 inches plus 0.25 inch, or 36.25 inches of board. So the board comes up short by a quarter inch. For a few cuts, the effect is small. For many cuts, it adds up: ten cuts with the same blade remove 10 × 0.125 = 1.25 inches.
Which side of the line?
Where you place the blade relative to your pencil line determines which piece is the correct size.
- Cut on the waste side of the line and leave the line on your piece. This is the usual approach. The kerf comes from the offcut.
- Cut on the line and the kerf takes material from both sides, so the piece may be slightly short.
- Cut on the far side and the piece will end up slightly long.
Many people mark an X on the waste side so there is no confusion. Marking with a knife line, rather than a wide pencil mark, gives a more accurate reference, since pencil lines have their own thickness.
When laying out a cut list for several parts from one board or sheet, add the kerf for each cut to your plan, and add a little extra to allow for trimming. A planning sketch that includes kerf lines is a useful habit.
Kerf and joinery
Some joints depend on matching the kerf. Cuts for grooves and dados may use a stack of blades or several passes, and the width of the slot should match the thickness of the part that goes into it. Miter cuts and bevels add complication, because the kerf is still the same width, but the angle changes how it affects the final length. For tight miters, accuracy depends on the setup, and the gaps in miter joints are often a result of angle and alignment problems. Kerf is one thing to keep in mind among several.
Kerf and blade choice
Blade thickness affects both the cut and the tool.
Thin-kerf blades:
- Remove less material, so they waste less of an expensive board and need less power to push through.
- Are popular on cordless and lower-power saws, where reducing the load can help.
- Can flex more, which may cause wandering in thick or hard material if the blade is not supported or the feed is too fast.
Standard or full-kerf blades:
- Are thicker and stiffer, and tend to stay flat.
- Remove more material, requiring more power.
- Are often a better match when stability matters, such as on a larger table saw.
Neither is simply better. A thin blade used with a stabilizer or a well-set saw can work well, and a standard blade suits applications where stiffness is important. Check the saw’s manual for recommended blade thickness. Some saw guards and riving knives are designed for a particular range of blade thicknesses, and a mismatch can be unsafe. This is one of many differences among saw blade types.
Kerf and other materials
Kerf applies to more than wood. Metal cutoff wheels, plasma cutters and laser cutters each produce a kerf, and in some processes the kerf width varies with settings. In sheet goods and in metal, the same planning principle applies: allow for the material the process removes.
Quick habits
- Check the blade’s printed thickness or measure the cut on scrap.
- Mark the waste side of the line.
- Add kerf to cut lists.
- Use a stop block for repeated pieces so every part is the same, regardless of kerf.
- Wear eye and hearing protection, and unplug or remove the battery before changing a blade.