What Is Kerf in Laser Cutting (and How to Account for It)
Kerf is the width of material the laser beam vaporises as it cuts, so a cut part comes out very slightly smaller than the line you drew and a hole comes out slightly larger. On most desktop lasers the kerf is roughly 0.1 to 0.3 mm. For a single decorative shape it does not matter, but for parts that must fit together, boxes, inlays, gears, you compensate for the kerf so the joints are tight.
Why kerf exists
A laser cuts by burning away a thin channel of material along your path. That channel has width, the kerf, because the focused beam is not infinitely thin and it removes material on both sides of the line. So the cut does not land exactly on your vector; it straddles it, taking roughly half the kerf from each side. The result: outside cuts (parts) come out a hair smaller than drawn, and inside cuts (holes and slots) come out a hair larger.
When kerf matters and when it does not
For a standalone shape, a coaster, a sign, an ornament, kerf is invisible; a fraction of a millimetre off the outer edge does not change anything. Kerf matters when parts have to fit each other: finger joints on a box, a peg into a hole, an inlay into a pocket, teeth on interlocking gears. There, half a kerf on each mating edge adds up to a loose, rattly fit unless you account for it.
How to measure your kerf
Kerf depends on your laser, lens, material, and settings, so measure your own. Cut a small square from a sheet, then measure the leftover hole and the cut-out piece with calipers. The difference between the drawn size and the measured piece, divided across both sides, gives your kerf. A quicker method: cut a comb of slots and find which slot a known-thickness offcut fits snugly, the difference tells you the kerf for that material.
How to compensate
Once you know your kerf, apply a kerf offset. For parts that need a tight fit, expand outer cut lines outward by half the kerf and shrink inner cut lines (holes) inward by half the kerf, so the finished dimensions match your intended size. Many laser programs (LightBurn included) have a kerf offset setting that does this automatically per cut layer. For press-fit joints, some makers deliberately leave a touch of the kerf uncorrected so the joint grips.
Kerf in your design software
You can also bake a small allowance into the design itself, for example sizing a slot to your material thickness plus a hair, rather than adjusting cut lines. However you do it, the principle is the same: the cut removes material, so mating parts need to be drawn slightly oversized (parts) or undersized (holes) to end up the right size after the beam takes its share.
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Convert a design with mm sizing and kerfFrequently asked questions
What is kerf in simple terms?
Kerf is the width of material the laser beam burns away as it cuts. Because it has width, cut parts come out slightly smaller than drawn and holes come out slightly larger, usually by about 0.1 to 0.3 mm on a desktop laser.
Do I always need to compensate for kerf?
No. For standalone decorative shapes it does not matter. Compensate only when parts must fit together, box joints, pegs, inlays, gears, where half a kerf on each edge adds up to a loose fit.
How do I measure my kerf?
Cut a small square, then measure both the leftover hole and the cut-out piece with calipers. The difference between drawn and measured size, split across both sides, is your kerf for that material and settings.
How do I apply kerf compensation?
Expand outer cut lines by half the kerf and shrink inner cut lines (holes) by half the kerf. Many laser programs have a per-layer kerf offset setting that does this for you.
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