If you have ever cut two pieces that were supposed to fit together and found they were slightly loose or slightly too tight — kerf offset is why. Understanding and accounting for kerf is the difference between parts that fit perfectly and parts that need sanding, shimming, or a second cut.
Kerf is the width of material removed by the laser as it cuts. The laser beam has a finite width — typically 0.1mm to 0.3mm on a diode laser and 0.2mm to 0.5mm on a CO2 laser — and everything within that beam path gets vaporized. The result is that a cut piece is slightly smaller than the design file, and the hole it leaves behind is slightly larger.
For decorative engraving and simple cuts where fit doesn't matter, kerf is irrelevant. But for box joints, press-fit assemblies, interlocking parts, and anything where two pieces need to fit together precisely, kerf is critical.
The most accurate way to measure kerf is to cut a test piece. Cut a 100mm square out of your material, then measure the actual size of the cut piece with calipers. If the piece measures 99.7mm, your kerf is 0.3mm — the laser removed 0.15mm from each side of the cut path.
A more precise method: cut a stack of 10 identical strips from the same material, measure the total width of the stack, subtract from what 10 strips should measure, and divide by 10. This averages out measuring error for a more accurate kerf value.
In LightBurn, kerf offset is applied per layer in the Cut Settings Editor. Open your layer settings, scroll down to find "Kerf Offset", and enter half your measured kerf value. LightBurn applies this offset to expand or shrink the cut path to compensate for material removal.
For a cut-out piece that needs to be the correct size, use a negative kerf offset to shrink the cut path inward. For a hole that needs to be the correct size, use a positive kerf offset to expand the cut path outward. The Hobbyist Laser Calc includes a kerf offset starting value based on your machine type.
| Machine Type | Typical Kerf | Half-Kerf Offset |
|---|---|---|
| Diode laser (5–10W) | 0.10–0.20mm | 0.05–0.10mm |
| Diode laser (20W) | 0.15–0.25mm | 0.075–0.125mm |
| CO2 laser (40W) | 0.20–0.35mm | 0.10–0.175mm |
| CO2 laser (80W+) | 0.25–0.50mm | 0.125–0.25mm |
These are starting estimates. Actual kerf varies with material, focus quality, speed, and power. Always measure your actual kerf for precision work rather than relying on estimates.
Kerf matters whenever you are cutting parts that need to fit together — box joints, finger joints, press-fit assemblies, living hinges, and interlocking puzzles. For these applications, always measure your kerf and apply the offset before cutting your final material.
For decorative work, signs, and artwork where dimensions are not critical, kerf can generally be ignored. The difference of 0.2mm on a decorative piece is not visible to the eye.
The Hobbyist Laser Calc includes kerf offset estimates for your machine and material
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