How to Measure Kerf and Compensate for It
To measure kerf, cut a square of known size (for example 20 mm), measure it with a caliper and subtract: the kerf is the designed size minus the measured size. Typical values are about 0.1–0.25 mm in 3 mm plywood and wider on thicker or slower cuts. Compensate by offsetting your outlines by half the kerf, or use a generator that has a kerf setting.
You cut two pieces that should click together, and one of them just rattles. Or the opposite: you are leaning on a plywood tab with a mallet, fearing the whole box will crack. Both situations have one culprit: the thin strip of material the laser burned away, called the kerf. Once you know its width, fit problems become a number you can fix.
This guide shows how to measure kerf in about ten minutes with a caliper, how to read the result, and how to compensate for it. If you need the basics first, start with what kerf is.
What you need to measure kerf
You do not need fancy gear.
- A digital caliper (0.01 mm resolution). A cheap one is fine.
- An offcut of the exact material you will use, same thickness, same sheet.
- Your usual cutting settings from your LightBurn settings so the kerf you measure is the kerf you will get.
- LightBurn or any program where you can draw a few rectangles.
| Tool | Why | Skip it? |
|---|---|---|
| Digital caliper | Reads 0.01 mm | No, essential |
| Offcut of your real material | Kerf changes with material | No |
| Square or ruler | Only for sanity checks | Yes |
| Sandpaper | Remove char before measuring | No |
Method 1: the square test (fastest)
- Draw a square exactly 20.00 mm × 20.00 mm. Check the size in the shape properties.
- Cut it with your normal settings for that material.
- Remove the square and lightly rub the edges with sandpaper to take off char and fuzz, which would add false thickness.
- Measure the square across at three places and take the average. Say you get 19.84 mm.
- Kerf = designed size minus measured size = 20.00 - 19.84 = 0.16 mm.
Why does it work? The laser's beam is centred on your line, so it eats half a kerf on each side of an outside cut. A square loses half a kerf on both sides, so it ends up exactly one kerf smaller.
For better accuracy, cut five squares and average. The variation between them tells you how repeatable your machine is.
Pro tip: Measure the square across and also the hole it came out of. For an outside cut, the part shrinks by a kerf and the hole grows by a kerf. If your two numbers disagree a lot, your laser or focus is not consistent.
Method 2: the slot test (best for fit)
This one measures exactly what you care about: how tight a joint will be.
- Draw a row of ten slots, each 3.00 mm wide for 3 mm plywood, in steps: slot widths of 2.80, 2.85, 2.90 and so on up to 3.25 mm.
- Cut them in your plywood strip, and also keep a strip of the same plywood to use as the test tab.
- Push the tab into each slot. The first slot where it goes in with light finger pressure and stays put is your friction fit.
- Write the slot width down. That is the width where your kerf and your plywood thickness agree.
| Slot feels like | What it means |
|---|---|
| Needs a hammer or splits the wood | Too tight, slot too narrow |
| Tab goes in with firm finger pressure and stays | Good friction fit |
| Tab slides and falls out | Too loose |
Measure the actual plywood thickness with the caliper too. Nominal 3 mm plywood is often 2.8–3.2 mm in reality, and that difference is as large as the kerf itself.
Common mistake: Trusting the label on the sheet. A "3 mm" sheet can be 2.8 mm or 3.2 mm, and an entire box can be a loose fit simply because the plywood is thinner than the design assumed.
Typical kerf values
Your own measurement always wins, but these ranges help you sanity-check what you get.
| Setup | Typical kerf |
|---|---|
| CO2 40–100 W, 3 mm plywood, good focus | about 0.1–0.2 mm |
| Diode 10–20 W, 3 mm plywood | about 0.15–0.3 mm |
| 6 mm plywood, any laser | often wider, 0.2–0.4 mm, with taper |
| Acrylic on CO2 | about 0.1–0.2 mm |
Kerf is not a constant. Slower speeds, more power, bad focus and more passes all widen it. Thick material also tapers, so the top is wider than the bottom.
How to compensate for kerf
You have your number. Now choose how to use it.
Parts that must fit into holes or slots (box tabs, stands): Offset the outline outward by half the kerf, so after the laser eats the extra half kerf, the part ends at exactly the nominal size. Slots shrink slightly in the file for the same reason.
In LightBurn: select the closed shape, open Tools, Offset Shapes, enter half the kerf (0.08 mm for a 0.16 mm kerf), choose the direction (outward for parts, inward for holes), and apply. You can try it on a copy first.
In a generator: our finger joint box generator has a kerf slider from 0 to 0.4 mm. Enter the number you measured and it offsets the outlines outward by half the kerf for you. That is the easiest way if you make boxes. The full walkthrough is in how to make a laser cut finger joint box.
| Situation | What to do | Offset |
|---|---|---|
| Outer outline of a part that must be exact | Grow it | + kerf / 2 |
| Hole that must fit a part | Shrink it | - kerf / 2 |
| Decorative ornament | Ignore kerf | none |
| Press-fit tabs | Use kerf or the slot test result | + kerf / 2 |
Check your result with a test fit
After compensation, cut a tiny joint (two small pieces with one tab and one slot) and test the fit before you cut a whole project. If the fit is still too tight, add 0.02–0.05 mm to the kerf setting and try again. If it is too loose, reduce it. Write the final number and the material on a note, together with speed, power and focus.
Pro tip: Keep a kerf log: material, thickness, speed, power, passes, kerf. After three or four entries you will know your machine better than any datasheet.
Kerf and taper on thicker material
On 3 mm plywood the kerf is nearly the same at the top and bottom of the cut. On thicker sheets the beam is a cone, so the top of the slot is wider than the bottom. A 6 mm sheet can show 0.1 mm or more of difference between the two faces. Measure at the top and at the bottom and use the average for joints, or choose a focus point slightly inside the material to even it out. See how to focus a laser for the focus side.
| Material thickness | Taper you can expect | What to do |
|---|---|---|
| 3 mm | Barely measurable | Single kerf value is fine |
| 4–6 mm | Small, noticeable on tight joints | Use the average, test fit |
| 8–10 mm | Clear taper | Expect to sand or use looser fits |
How often to re-measure
Kerf changes when you change anything about the cut, so re-measure when you swap lens, change focus, move to another sheet batch, or change speed and power. A good habit is a 20 mm test square at the start of each project. It costs a minute and a few square centimetres of plywood.
Next step
Cut a 20 mm square in your usual plywood today, measure it and write the kerf on a sticky note. Then try it with the box generator: enter that kerf, cut one small box, and enjoy the satisfying click when the first joint closes.
Questions and answers
What is a normal kerf for a laser cutter?
In 3 mm plywood the typical kerf is about 0.1–0.25 mm. Diodes are often at the wider end because of a larger spot and slower speeds, while a well-focused CO2 can be around 0.1–0.2 mm. Always measure your own.
Does kerf change with material?
Yes. Thicker material, slower speed, more passes, lower focus quality and different woods all change the kerf. Measure for each material and thickness you use for tight-fitting parts.
Do I need to compensate kerf for ornaments?
Usually not. A tenth of a millimetre does not matter on a hanging ornament. It matters for boxes, slot-together models and any parts that must press-fit.
Should I measure kerf with a ruler?
A ruler is too coarse. Use a digital caliper with 0.01 mm resolution and measure several times, because plywood thickness and cut edges are not perfectly uniform.
Does kerf compensation work in LightBurn?
Yes. Use the Offset Shapes tool to grow or shrink a closed outline by half the kerf. Make sure you offset in the right direction for parts versus holes.