guides · technique · precision cutting
Laser Kerf: How to Measure It and Set the Offset in LightBurn
A diode laser removes roughly 0.15–0.40 mm of material along every cut line — that gap is the kerf. For engraving and simple decorative cuts, kerf is irrelevant. For box tabs, press-fit joints, or any project where two cut pieces must interlock, you need to measure your machine's kerf and enter it as a Kerf Offset in LightBurn (Cut Settings Editor → Line mode) so your cuts land exactly where the design intends, not half a millimetre short.
- Diode laser kerf on 3mm wood: 0.20–0.35 mm without air assist; 0.15–0.28 mm with air assist running
- Diode beams are rectangular, not circular — horizontal and vertical cuts differ by 0.05–0.10 mm; LightBurn uses one value for both
- LightBurn's Kerf Offset is in Cut Settings Editor → Line mode → "Kerf Offset" field — enter half your measured full kerf width
- LaserGRBL has no kerf offset — use Inkscape's Path > Outset/Inset on the design file instead
What is laser kerf and when does it actually matter?
When a laser cuts through material, the beam vaporises and expels a thin strip of the material along the cut line. That strip — the gap left behind — is the kerf. If you draw a line in your design software and tell the laser to cut it, the resulting physical cut is wider than an infinitely thin line by the kerf width: roughly 0.15–0.40 mm for a diode laser, depending on the material, the wattage, and whether air assist is running.
For most engraving work and decorative cuts, kerf is invisible in the finished result and you can safely ignore it. It starts to matter the moment two cut pieces need to interact:
- Finger joints and box tabs — the tab and slot must be the same width. Without a kerf offset, each tab is 0.15–0.40 mm undersized and the box rattles or falls apart.
- Press-fit assemblies — two pieces that should interlock by friction. Kerf makes them slip through each other instead.
- Inlays and stencils — a cut piece must drop into a cut hole exactly. Even 0.2 mm difference is visible as a gap or causes it to jam before seating flush.
- Repeatable sizing — if you're cutting 50 identical pieces and they need to match a specified dimension within ±0.3 mm, kerf matters.
Kerf does not matter for engraving, single-piece decorative cuts, or any situation where two cut pieces don't need to fit together. If you mostly engrave coasters or cut simple shapes to decorate, you can skip this guide entirely.
Why diode laser kerf is trickier than CO2
A CO2 laser produces a nearly circular beam. The kerf it leaves is consistent in all directions, which makes measurement and compensation straightforward. Diode lasers are different. The laser diode chip fires from the edge of a stacked semiconductor structure, producing a beam that is roughly rectangular in cross-section — wider in one axis than the other.
In practice, this means the kerf your diode leaves when cutting a horizontal line is slightly different from the kerf when cutting a vertical line — typically by 0.05–0.10 mm on most home machines. LightBurn's Kerf Offset field takes a single value and applies it equally in all directions. This is a known limitation of diode laser kerf compensation, and the practical workaround is:
- Measure kerf in both X and Y on your machine
- Use the larger value in LightBurn — this ensures nothing ends up too tight (slightly too loose is better than too tight for joints)
- For high-precision inlays, rotate the entire design 45 degrees on the bed — this distributes the X/Y directional bias symmetrically across cut edges
Typical kerf widths by material and wattage
These are estimated starting points based on published community data and typical diode laser spot sizes. Your machine will differ — always measure (see the next section). All values assume sharp focus and the nominal settings for each wattage class.
| Material | Thickness | 10W (no AA) | 10W (with AA) | 20W (with AA) | Notes |
|---|---|---|---|---|---|
| Basswood | 3 mm | 0.22–0.32 mm | 0.16–0.25 mm | 0.15–0.22 mm | Straight, clean grain; narrowest kerf of common woods |
| Birch plywood | 3 mm | 0.25–0.38 mm | 0.20–0.30 mm | 0.18–0.27 mm | Glue layers can flare; measure after verifying clean penetration on all plies |
| MDF | 3 mm | 0.30–0.45 mm | 0.25–0.38 mm | 0.22–0.35 mm | Resin content creates char buildup at cut walls; kerf is widest and most variable |
| Pine | 3 mm | 0.25–0.40 mm | 0.20–0.32 mm | 0.18–0.28 mm | Resin pockets flare cut width unpredictably; not ideal for precision joinery |
| Coloured acrylic | 3 mm | 0.18–0.28 mm | 0.15–0.22 mm | 0.12–0.20 mm | Cleaner wall than wood; cast acrylic gives more consistent kerf than extruded |
| Cork | 3 mm | 0.20–0.30 mm | 0.18–0.26 mm | 0.15–0.22 mm | Compressible — measuring cork kerf with calipers is less reliable; use the 10-square test |
Confidence: Medium — estimated. Source: community-reported ranges (LightBurn forum, diode-laser-wiki.com) cross-referenced with typical 450 nm diode spot sizes. Treat as starting points only — measure your machine.
How to measure your machine's kerf
There are two practical methods. The 10-square method is easy to understand and only requires a ruler. The LightBurn Vernier test is more accurate and works without a micrometer.
Method A: The 10-square method (no LightBurn required)
- In your design software, draw 10 identical squares at a known size — 50 × 50 mm is convenient. Arrange them in a single row.
- Cut the squares with your kerf offset set to 0.
- Push all 10 squares together end-to-end along their cut edges. Use a flat surface and a stop block to keep them aligned.
- Measure the total length of the row with a ruler or caliper. If 10 squares were perfectly 50 mm each, the row should be 500 mm. If it measures 497 mm, the shortfall is 3 mm across 10 squares, meaning each cut removed 0.30 mm.
- That 0.30 mm is your kerf width. Repeat the measurement perpendicular to the first run (cut 10 squares arranged vertically if the first row was horizontal) to get your Y-axis kerf.
Use the larger of the two measurements as your LightBurn Kerf Offset value.
Method B: LightBurn's Vernier Kerf Offset test (more accurate)
LightBurn ships a downloadable Vernier Kerf Offset test file that uses an internal scale to measure kerf without requiring a caliper accurate to 0.01 mm. The process:
- Download the Vernier Kerf Offset test from the LightBurn documentation site. Cut it with Kerf Offset set to 0.
- Slide the two cut pieces together to align the Vernier scale.
- Read measurement D and measurement E from the scale markings.
- Apply the formula:
(D.E) ÷ 40 = kerf offset. Enter the result directly into the Kerf Offset field.
This method is more accurate than the 10-square method but requires LightBurn. The 10-square method works fine for all practical joinery on home machines.
How to set the kerf offset in LightBurn
Once you have your measured kerf width, setting it in LightBurn takes about 30 seconds:
- In LightBurn, open the Cuts / Layers panel (usually docked on the right).
- Double-click the cut layer you want to adjust (the layer assigned to your cut paths, typically set to Line mode and a low power setting so it cuts through).
- The Cut Settings Editor opens. Look for the Kerf Offset field — it's in the same panel as speed and power, usually near the bottom.
- Enter the kerf offset value. Enter your full measured kerf width here — LightBurn automatically applies half the value to each side of the cut path, which is correct behaviour. So if your measured kerf is 0.30 mm, enter 0.30.
- Positive values make the cut path move outward from the shape (the cut piece gets smaller, which is correct for outer profiles). Negative values shrink the path inward.
- Click OK. Run a test cut of the same squares used for measurement. Measure the result: the cut squares should now be very close to 50 × 50 mm.
- Positive kerf offset — use for outer cuts (cutting a piece out of stock material). The path expands outward so the piece comes out at the designed size despite the kerf.
- Negative kerf offset — use for interior cuts (pockets, slots, holes). The path shrinks inward so the opening is the designed size, not larger by the kerf width.
- Many machinists set separate layers for outer profiles and inner pockets and give them equal and opposite kerf offsets.
Save the kerf offset as part of a named material preset in LightBurn's library (Window → Library). That way you never have to re-enter it — just select the material profile and the offset is already set.
What offset to use for press-fit vs slip-fit vs clearance
Not all joinery needs the same tightness. The three standard fit types, and how to achieve each with kerf offset, are:
| Fit type | What it means | Kerf offset to use | Typical use |
|---|---|---|---|
| Press fit | Pieces interlock by friction with light hand pressure; stays together without glue | Exactly your measured kerf width (no extra adjustment) | Box joints, snap-together assemblies, living hinges |
| Slip fit | Pieces slide together easily with no friction; light play | Measured kerf + 0.10–0.20 mm additional | Sliding lids, disassembly required, dimensional uncertainty in stock |
| Clearance fit | Obvious gap; easy insertion; looks intentional from the front | Measured kerf + 0.30–0.50 mm additional | Templates, stencils, decorative overlays where exact flush-fit isn't needed |
Wood is hygroscopic — it expands and contracts with humidity. For projects that will live outdoors or in a damp environment, design for a slip fit (+0.10–0.20 mm) to leave room for seasonal movement without the joint cracking.
Using LaserGRBL — no kerf offset built in
LaserGRBL does not have a kerf offset setting. You have two options:
Option 1: Apply the offset in Inkscape before exporting
- Open your design in Inkscape (free).
- Select all outer profile paths (the shapes you're cutting out).
- Go to Path → Outset (default outset is 2 px — you'll need to set it to match your kerf width). Use Path → Dynamic Offset for more control over the outset distance.
- For inner cuts (slots, holes), use Path → Inset instead.
- Export the adjusted design as SVG or DXF and import it into LaserGRBL as usual.
Option 2: Switch to LightBurn for precision cuts
If you're doing work that requires kerf offset regularly — boxes, inlays, joinery — this is the practical argument for upgrading from LaserGRBL to LightBurn. LightBurn's kerf offset is per-layer, persistent, easy to toggle, and saves with material presets. LaserGRBL is excellent for simple burns but doesn't have the precision tooling for joinery.
Why does kerf keep changing?
Kerf is not a fixed property of your machine — it shifts based on several variables:
- Focus accuracy — a 1 mm focus error on a 10W module can widen the kerf by 0.05–0.15 mm because the spot defocuses into a larger ellipse. Re-check focus after any hardware movement. See the focus guide.
- Material moisture content — damp wood expands slightly and burns with more charring, widening the kerf by 0.05–0.10 mm compared to dry stock. Measure fresh batches of plywood separately.
- Lens contamination — a dirty protective lens scatters the beam, defocusing it and widening the kerf. If cuts start requiring more passes, clean the lens first before adjusting settings. More troubleshooting here.
- Cut speed — faster speeds leave less energy per millimetre, resulting in a slightly narrower kerf (the beam has less time to heat the surrounding material). If you change speed significantly, re-measure kerf.
- Air assist pressure — higher air assist pressure blows expelled material more aggressively, narrowing the kerf slightly. Measure with the same air assist settings you'll use in production.
- Machine temperature — diode modules produce narrower kerf at optimal operating temperature. A module that's been running for 30 minutes is warmer than at first power-on; kerf on the first cut can be 0.02–0.05 mm wider. Warm up the machine for 2–3 minutes before cutting precision parts.
The practical rule: measure kerf on the same material, in the same conditions, with the same settings you'll use for the final project. A kerf measurement taken on one material or speed is not transferable to another without re-measuring.
- Digital calipers (150 mm range) — 0.01 mm resolution; accurate enough for kerf measurement when using the 10-square method
- Honeycomb cutting bed — flat, stable surface that supports cut pieces consistently and reduces flashback that can widen the kerf on the cut piece's underside
- OD7+ laser safety glasses (450 nm) — required any time the diode is powered; never run test cuts without proper eye protection
As an Amazon Associate I earn from qualifying purchases.
Frequently asked questions about laser kerf
What is a typical kerf width for a diode laser?
A 10W diode laser typically removes 0.20–0.35 mm of material per cut on 3mm wood. With air assist running, the kerf narrows slightly to 0.15–0.28 mm. The spot size — not the wattage — primarily determines kerf width. Always measure your specific machine, as beam quality and focus precision vary between machines and even between identical models.
Where is the kerf offset setting in LightBurn?
Open the Cut Settings Editor by double-clicking any layer in the Cuts/Layers panel. Set the mode to Line. The Kerf Offset field appears near the bottom of the settings. Enter a positive value to expand the cut path outward (making the cut piece smaller), or a negative value to shrink it inward. Enter your full measured kerf width — LightBurn splits it evenly across both sides of the cut path.
Why does my diode laser have different kerf widths in X and Y?
Diode laser beams are rectangular in cross-section (not circular), because the diode chip fires from the edge of a stacked semiconductor layer. This means horizontal and vertical cuts differ in kerf by 0.05–0.10 mm. LightBurn's single Kerf Offset value applies uniformly in all directions. Use the larger of your two measured values to ensure nothing ends up too tight. For maximum precision, rotate the entire design 45 degrees — this distributes the directional bias symmetrically across all edges.
Does LaserGRBL have a kerf offset setting?
No. The workaround is to apply the offset in Inkscape before exporting: use Path → Outset for outer profiles and Path → Inset for interior cuts. This achieves the same result as LightBurn's kerf offset but must be done in the design file rather than at cut time.