What Is Kerf in Laser Cutting?
Kerf is the width of material a laser burns away along a cut line. In 3 mm plywood it is typically 0.1-0.25 mm, depending on the laser, focus, speed and material. Kerf matters when parts must fit together, such as finger joints, because the cut shape ends up slightly smaller than the drawn line.
You cut a box, slide the sides together, and they either fall apart in your hands or refuse to close without a mallet. Welcome to kerf, the invisible sliver of wood your laser turned into smoke.
Kerf is the width of material removed by the laser beam as it cuts. If you could look at the cut edge from above, the gap between the two pieces is the kerf.
Typical kerf values
Kerf is not a fixed number. It depends on the beam spot size, focus, speed, power, air assist and material.
| Material and laser | Typical kerf |
|---|---|
| 3 mm plywood, diode laser | about 0.15-0.30 mm |
| 3 mm plywood, CO2 laser | about 0.10-0.25 mm |
| 3 mm cast acrylic, CO2 | about 0.10-0.20 mm |
| 6 mm plywood, CO2 | about 0.20-0.35 mm |
| Cardboard, paper | about 0.05-0.15 mm |
These are ranges, not promises. Measure your own on scrap (see How to Measure Kerf and Compensate for It).
What affects kerf
- Focus. A beam that is in focus makes the narrowest kerf; defocus widens it.
- Speed and power. Slower and hotter cuts burn a wider channel.
- Material. Resinous or glue-heavy wood chars more than clean birch.
- Thickness. The beam tapers, so thicker material has a slightly wider top and narrower bottom.
- Air assist. Strong air blows away debris and can tighten the kerf.
Does kerf matter for your project?
It depends on whether pieces must fit.
| Project | Does kerf matter? |
|---|---|
| Ornament, sign, plaque | No, a 0.2 mm difference is invisible |
| Cutting a shape out of a frame | A little, the hole is larger than drawn |
| Finger-joint box | Yes, joints get loose by one kerf per fit |
| Interlocking puzzle or slot-together model | Yes, critical |
Common mistake: Measuring kerf on one cut and applying it everywhere. Kerf changes with material thickness, speed and even the day's focus, so re-measure whenever you change any of them.
Worked example
You cut a 3 mm plywood tab drawn exactly 3.00 mm wide, and the slot also drawn 3.00 mm. Your kerf is 0.2 mm. The tab, being an outer shape, comes out 2.80 mm wide, and the slot, being a hole, comes out 3.20 mm wide. The gap between them is 0.40 mm, which makes the joint loose. To fix it, offset the tab outward by half the kerf (0.1 mm per side, so draw it 3.20 mm) and the slot inward by 0.1 mm per side (draw it 2.80 mm). Our finger joint box generator has a kerf setting from 0 to 0.4 mm, so you can do exactly that automatically.
Pro tip: If your joint is slightly loose, leave the kerf compensation at zero and try 0.05-0.1 mm steps until it presses together with firm finger pressure.
Where kerf shows up next
Kerf is linked to other topics: speed and power affect it (LightBurn Settings for 3 mm Plywood), and a good box needs it set right (How to Make a Laser Cut Finger Joint Box).
Kerf versus tolerance
People often blur two ideas. Kerf is how much the laser removes. Tolerance is how much play you want in the fit. A press fit might want zero or slightly negative clearance, while a sliding lid wants 0.1-0.2 mm of slack. Wood also swells with humidity, so a joint that is perfect in a dry workshop can bind in a damp room. Measure kerf first, then choose your fit on top of it.
| Fit you want | Clearance after kerf is applied |
|---|---|
| Tight press fit (glued) | about 0 to 0.05 mm |
| Snug hand-pressed joint | about 0.05-0.1 mm |
| Sliding lid or drawer | about 0.15-0.3 mm |
Next step
Cut a ten-piece test strip of 3 mm plywood with slots of increasing width, find the one that grips your tab best, and write it down. Then use it in the box generator.
Questions and answers
What is a normal kerf for a CO2 laser?
About 0.10-0.25 mm in 3 mm plywood. It varies with focus, speed, power and material.
Is kerf the same for engraving?
No. Kerf only describes cut-through lines. Engraving removes a shallow area and has its own line width.
Why are my box joints loose?
The laser removed material (kerf) from both parts, so each joint gets looser by about one kerf. Apply kerf compensation or use a generator that does.
Does a diode laser have more kerf than CO2?
Often slightly more, because slower speeds mean more heat spreads into the wood, though both are narrow.
How do I measure kerf?
Cut a known-size square or slot on scrap, measure the result with calipers and compare it with the drawing. Half the difference per side is your offset.
Do I need to adjust kerf for ornaments?
No. For decorations that do not interlock, a tenth or two of a millimetre is not noticeable.