Laser kerf & joint fit

Why joints come out loose

The beam does not cut a line of zero width. It burns away a strip — the kerf — centred on the line you drew. How much looser a joint gets depends on how many of its faces are cut:

• Fingers and notches (box joints): the finger loses half a kerf on each side, so it is one kerf narrower than drawn, and the notch it goes into is one kerf wider. The gap is about two kerfs.
• A slot for the sheet’s thickness (tab-and-slot, dividers): only the slot is cut. The sheet’s own thickness is not, so the gap is about one kerf.

Moving every cut line outward by half the kerf on outlines, and inward on holes, puts the burned strip outside the part, so the part ends up the size you drew. Most software does this for you when you enter the offset; the trick is knowing whether a tool wants the full kerf or half of it.

Three ways to measure kerf

1. Cut a square. Cut a 20 mm square and measure the piece that falls out. Kerf = 20 − measured. Measure bare material, not masking tape, and avoid charred edges. Calipers struggle below about 0.1 mm, so use the next method if your kerf is small.
2. Comb of strips. Cut ten strips inside a frame (eleven cut lines), push the strips to one side and measure the total gap with feeler gauges or calipers. Kerf = gap ÷ number of cut lines. Ponoko built its kerf table this way.
3. LightBurn Vernier test. LightBurn’s kerf test reads off a Vernier scale: Kerf Offset = D.E ÷ 40. That value is already the offset per side.

Kerf can differ by direction. Diode spots are often not round, and one Fab Academy test on a CO2 laser measured 0.217 mm one way and 0.264 mm the other in MDF. If joints fit one way but not the other, measure an X cut and a Y cut separately.

What each tool means by “kerf”

The table in the calculator turns your measurement into the right value for each tool. The short version: LightBurn, xTool Creative Space, boxes.py and MakerCase want half the kerf (the offset per side). TabbedBoxMaker wants the full cut width and halves it internally. Cuttle’s documentation does not say which it means, so test both values with its free kerf tester. In the box generators (boxes.py, MakerCase, Cuttle, TabbedBoxMaker), a larger value gives a tighter joint.

In LightBurn, a positive Kerf Offset grows closed shapes outward, and its documentation says shapes inside other shapes on the same layer get the opposite direction — so holes and slots shrink and the joint tightens, as long as they share a layer with the outline. xTool’s Creative Space manual says the offset is in theory 0.5 × the spot size, while an older xTool Academy article says the offset equals the kerf width; we follow the manual. The XCS manual does not say how holes are treated and rounds the value to two decimals, so check a hole on a test cut.

Published kerf widths

We found no per-material kerf table from Trotec, Epilog or Glowforge, and none for diode lasers from any maker. Use these only to sanity-check your own measurement.

Press fit or slip fit?

We found no standard that gives fixed allowances for laser-cut joints; every source we read says to cut a test piece. Wood compresses a little, so a press fit can work with the offset at exactly half the kerf or slightly more. Acrylic does not compress and can crack, so leave it a hair loose and glue it. Plywood sheets also vary in thickness, so measure the sheet you are cutting, not the nominal 3 mm.

FAQ

How do I measure laser kerf?

Cut a square of known size, for example 20 mm, and measure the piece that falls out with calipers. The kerf is the design size minus the measured size. For more accuracy, cut a row of strips inside a frame, push them together, measure the total gap and divide it by the number of cut lines.

Is LightBurn Kerf Offset the full kerf or half?

Enter the offset per side, which is half the measured kerf. LightBurn’s own Vernier kerf test gives a value (D.E ÷ 40) that you enter as-is; that number is already per side. Positive values grow closed shapes outward.

Why are my laser-cut finger joints loose?

Because the beam burns away material on both sides of every line. Without compensation a finger comes out one kerf narrower and its notch one kerf wider, so a finger joint has a gap of about twice the kerf; a slot that takes a sheet’s thickness opens by about one kerf. Offset the cut path outward by half the kerf on outlines and inward on holes.

What is a typical laser kerf?

Roughly 0.1–0.3 mm on hobby and small CO2 lasers for 3 mm acrylic, plywood and MDF. Ponoko lists 0.125 mm for 3 mm acrylic and 0.20 mm for 3 mm plywood on a 40 W CO2 laser, and its current help says kerf starts at about 0.2 mm. Measure your own: it changes with focus, power, speed and material.

Sources

• LightBurn, Line mode settings and Kerf offset test (Vernier reading, and the offset direction inverting for shapes inside other shapes on the same layer).
• xTool support, Kerf offset in xTool Creative Space; xTool Academy, Laser cutting kerf.
• boxes.py, BurnTest and source code (burn parameter).
• TabbedBoxMaker, GitHub repository (Kerf = cut width).
• University of Victoria machine shop, MakerCase guide (the field is half the kerf).
• Cuttle, Laser-cut boxes.
• Ponoko, How much material does the laser burn away? and Figuring out kerf for precision parts.
• Fab Academy, group kerf tests.

Last checked 2026-10-27 · Calculated values; no machine was used to make this page. See also the line interval & DPI calculator.