Kerf is the width of material that a laser removes as it cuts. Every laser cut vaporizes or melts a thin strip of material, leaving a gap between the two separated pieces. That gap is the kerf, and it typically ranges from 0.08mm to 1mm depending on the material, its thickness, and the cutting parameters. Understanding kerf matters because it directly affects whether your finished parts come out at the dimensions you designed.
Why Laser Cuts Have a Width
A laser beam isn’t infinitely thin. It’s a focused cone of energy with a measurable spot diameter, and when it hits material, it heats a small zone beyond that spot through thermal conduction. The kerf ends up slightly wider than the beam’s spot size because of this thermal spread. In practical terms, if you laser-cut a 6-inch square without accounting for kerf, your finished piece will be slightly smaller than 6 inches on each side, because the laser consumed material along every edge.
Laser cutting produces one of the narrowest kerfs of any cutting technology. Plasma cutting, by comparison, can remove 5mm or more of material per cut. Waterjet is narrower than plasma but still wider than laser. This small kerf is one of the main reasons laser cutting achieves such tight tolerances.
Typical Kerf Widths by Material
Kerf varies significantly with material type and thickness. Thicker materials require more energy and produce wider cuts. Here are measured averages for CO2 laser cutting:
- Acrylic, 1–3mm thick: 0.18mm kerf
- Acrylic, 5–8mm thick: 0.21mm kerf
- Acrylic, 10–15mm thick: 0.30mm kerf
- Acrylic, 20mm thick: 0.32mm kerf
- Birch plywood, 0.8mm thick: 0.08mm kerf
- Birch plywood, 1.5mm thick: 0.16mm kerf
- Birch plywood, 3mm thick: 0.20mm kerf
- Birch plywood, 6mm thick: 0.22mm kerf
- Birch plywood, 12mm thick: 0.30mm kerf
The pattern is consistent: doubling the material thickness doesn’t double the kerf, but it does increase it noticeably. A thin sheet of plywood loses less than a tenth of a millimeter per cut, while thick acrylic loses about a third of a millimeter.
What Controls Kerf Width
Several variables determine how wide your kerf ends up. The two most influential are laser power and cutting speed. Higher power delivers more energy to the material, melting or vaporizing a wider zone. Faster cutting speed reduces the time the beam dwells on any one spot, which narrows the kerf. In practice, these two settings are balanced against each other to get a clean cut without excessive material removal.
The focal point position is another critical factor. The laser beam converges to its tightest point at a specific distance from the lens, and the vertical position of that focal point relative to the material surface directly controls the spot size hitting the workpiece. Research from Politecnico di Milano demonstrated that the relationship between focal position and kerf width is almost perfectly linear, with a statistical fit above 99%. Even small shifts in focus, whether from thermal drift during a long cutting job or incorrect setup, change the kerf width and can degrade edge quality or cause failed cuts.
Other factors that play a role include the standoff distance (gap between the nozzle and material), nozzle diameter, and pulse settings on pulsed lasers. Thinner materials, smaller nozzle openings, and medium pulse frequencies all tend to produce narrower kerfs.
How Assist Gas Affects the Cut
Laser cutters blow a stream of gas through the nozzle alongside the beam. This assist gas serves two purposes: it clears molten material out of the cut and it influences the chemistry of the cutting process. The type of gas you choose changes both kerf width and edge quality.
Oxygen is commonly used for cutting mild steel. It reacts with the heated metal in an exothermic reaction, adding energy to the cut and increasing cutting speed. The tradeoff is that this extra heat can cause side burning, especially in thicker sections, which widens the kerf and leaves an oxidized edge. Oxygen pressures are kept relatively low (around 0.75 to 2 bar for 3mm mild steel) to control this effect.
Nitrogen produces cleaner, oxide-free edges because it doesn’t react with the material. It’s used at much higher pressures (5 to 15 bar) to blow molten material out of the kerf mechanically. This makes it the preferred choice when cut quality and appearance matter more than raw speed. For aluminum alloys, argon can outperform nitrogen because its higher density makes it more effective at clearing the cut zone, and it avoids forming nitrides that increase melt viscosity and reduce cut quality.
Kerf Taper: Top vs. Bottom Width
Laser kerf isn’t perfectly uniform from top to bottom. The beam is most focused at one plane, and it diverges above and below that point. This means the top of the cut is often slightly wider than the bottom, or vice versa, creating a subtle taper on the cut edges. The taper angle is small, often well under one degree, but it can matter for parts that need to fit together precisely or for thick materials where the effect is more pronounced.
Optimizing the focal point position is the main way to minimize taper. Placing the focal point at the right depth within the material distributes the beam energy more evenly through the full thickness of the cut.
Compensating for Kerf in Your Design
If you design a part at exactly the dimensions you want and send it to a laser cutter without adjustments, every edge will be smaller than intended by half the kerf width. The laser follows the center of your design path, removing material equally on both sides. For a 0.4mm kerf, that means you lose 0.2mm from each edge.
Kerf compensation fixes this by shifting the laser’s toolpath to account for the material it removes. The logic is straightforward:
- Cutting out a part (outside cut): Offset the toolpath outward by half the kerf width. For a 50mm square with 0.4mm kerf, the laser follows a 50.4mm path so the finished part measures exactly 50mm.
- Cutting a hole (inside cut): Offset the toolpath inward by half the kerf width. This ensures the hole ends up at the correct diameter rather than oversized.
Many laser cutting software packages handle this automatically. You enter your measured kerf value once, and the software applies the correct offset to every cut. If your software doesn’t support automatic compensation, you can do it manually. In Inkscape, use Path, then Dynamic Offset to drag the path larger or smaller by half your kerf measurement. In Illustrator, use Object, then Path, then Offset Path, entering a positive value for outward offsets and negative for inward.
How to Measure Your Kerf
Published kerf values are useful starting points, but your specific machine, lens, material batch, and settings will produce a kerf that’s unique to your setup. The most reliable approach is to cut a test piece and measure the gap with digital calipers. Cut a simple rectangle, measure the rectangle, and compare it to your design dimensions. The difference on each side is half the kerf.
For even more accuracy, cut a series of closely spaced parallel lines and measure the total width removed across all cuts. Dividing by the number of cuts averages out any measurement error. Once you have a reliable kerf number for a given material and thickness at your usual settings, save it as a preset. You only need to remeasure when you change materials, thickness, or significantly adjust power and speed.