Laser Kerf Explained: Why Your Parts Fit Loose (and the Fix)

You draw a 20 mm tab and a 20 mm slot, cut both, and the tab drops through with room to spare. Nothing is wrong with your drawing. The culprit is laser kerf: the thin strip of material the beam turns to smoke as it cuts. It is small, usually a fraction of a millimetre, but for anything that has to fit together it decides between a joint that holds and one that rattles.

What is laser kerf?

Kerf is the width of the cut. A laser beam is focused to a tiny spot, but that spot still has width, and the heat spreads a little into the material around it. As the head travels along your line, it removes a channel of material. The width of that channel is the kerf.

On a typical CO2 laser cutting 3 mm wood or acrylic, kerf is often somewhere around 0.1 to 0.25 mm. Diode lasers vary a lot with module and focus. Your own machine may sit outside these ranges, which is why measuring it matters.

Why kerf changes the size of your parts

The laser follows the centre of your line. Half the kerf falls on one side of the line and half on the other.

• An outer part (a tab, a coaster, a box side) loses half a kerf on every edge, so it ends up one full kerf smaller than drawn in each direction.
• A hole or slot gains half a kerf on every edge, so it ends up one full kerf larger than drawn.

Put those together and a tab that should fit a slot exactly now has a gap of two kerf widths, one from each piece. With a 0.15 mm kerf, that is 0.3 mm of play. In a box with dozens of finger joints, it adds up to a wobbly box held together by glue alone.

What affects kerf width

Kerf is not one fixed number for your machine. It shifts with:

• Material. Woods, acrylic, card and leather each burn differently. Denser material often gives a narrower, cleaner kerf.
• Thickness. Thicker sheets need more energy, and the beam spreads as it goes deeper, so kerf grows with thickness.
• Power and speed. More power or slower speed puts more heat in and widens the cut.
• Number of passes. Each extra pass tends to widen the kerf slightly.
• Focus and lens. A correctly focused, short focal length lens gives the narrowest spot. A dirty or out-of-focus lens gives a wider, messier cut.
• Air assist. Good airflow clears smoke and limits charring, keeping the kerf tidy.

There is also taper. The beam is cone-shaped around its focal point, so on thicker material the cut is often wider at the top than at the bottom. That is why one face of a cut part sometimes fits more tightly than the other. Flipping a part can change how it fits.

Typical laser kerf starting values

Here are the starting values the Digital Creation laser cut tool uses for 3 mm material on a CO2 laser. Treat them as a sensible first guess, then confirm them on your own machine:

• Plywood: 0.15 mm
• Birch plywood: 0.15 mm
• MDF: 0.15 mm
• Basswood: 0.12 mm
• Cast acrylic: 0.12 mm
• Cardboard: 0.12 mm
• Veg-tanned leather: 0.12 mm

Thicker material generally needs a larger value. If you move from 3 mm to 6 mm plywood, expect the kerf to grow, and test again rather than reusing the 3 mm number.

How to compensate for kerf

There are three common ways to allow for kerf. Use one, not several, or you will double the correction.

1. Build it into the design

The designer makes outer parts bigger by half the kerf on each edge, and holes smaller by half the kerf on each edge. Tabs come out at their true size and slots come out at their true size, so they fit. This is how most parametric box generators work, and it is how the files from the laser cut file generator handle joints: kerf is allowed for in the joints when the file is made.

2. Use your software's kerf offset

LightBurn has a Kerf Offset setting on each cut layer. The value is applied to each edge, so it is normally half your measured kerf. Enter it, and the software shifts the cutting path outward for outer shapes and inward for holes. xTool Creative Space and some other programs have similar options. This is handy when you are cutting a design that was drawn at exact size.

3. Offset the paths yourself

In Inkscape you can use Path > Outset or Linked Offset; in Illustrator, Object > Path > Offset Path. Offset outer shapes outward and holes inward by half the kerf. It works, but it is slow on complex designs and easy to get wrong.

If a file already includes kerf compensation, do not add a kerf offset in your laser software as well.

Choosing the fit you want

Kerf compensation is also how you tune the feel of a joint.

• Press fit: compensate with the full measured kerf, or slightly more. Parts push together firmly and hold without glue, which suits boxes and stands.
• Slip fit: compensate with a little less than the measured kerf. Parts slide together easily and are usually glued. This suits lids, drawers and parts you will take apart.
• Very loose: no compensation. Fine for decorations, but not for structural joints.

Wood moves with humidity and plywood thickness varies across a sheet, so a fit that is perfect in one batch may be slightly tight in the next. Leave room to adjust.

Common kerf problems and fixes

Joints too loose. Increase the kerf value in your design or offset. Also measure the sheet; thinner sheets make slots look loose because the slot width was designed for thicker material.

Joints too tight to assemble. Reduce the kerf value. Check whether you applied compensation twice, once in the file and once in the software.

One side fits, the other does not. That is taper. Try a better-focused cut, a shorter focal length lens, or set the focus slightly into the material on thick sheets.

Fit changes across the sheet. The sheet is warped or varies in thickness, or the bed is not level. Hold the sheet flat and check the focus at several points.

Burnt, wide cuts. Too much power or too slow. Clean the lens, check air assist, and run a material test.

Measure, do not guess

The values above will get most makers close. To get a fit you are happy with, measure your own kerf on the exact material and settings you plan to use. The step-by-step laser kerf test takes about ten minutes and one small offcut. For more on setting up jointed designs from scratch, see how to make laser cut files, and for the difference between cut, engrave and score layers, see laser cutting vs engraving vs scoring.

Frequently asked questions

How wide is a laser kerf?

On a CO2 laser cutting 3 mm sheet, kerf is often around 0.1 to 0.25 mm. It depends on the material, thickness, focus and settings, so measure your own.

Does kerf make parts smaller or bigger?

Outer parts come out smaller by about one kerf width, and holes come out bigger by about one kerf width, because the beam removes material on both sides of the line.

Do I need to allow for kerf on every design?

Only where parts fit together, such as finger joints, tab and slot joints and inlays. Signs, ornaments and coasters can be drawn at exact size.

Is kerf the same for every material?

No. Each material burns differently, and thicker sheets usually have a wider kerf. Keep a note of the kerf for each material and thickness you use.