How to Do a Laser Kerf Test in 10 Minutes (Step by Step)

A laser kerf test is the cheapest ten minutes you will spend on your machine. It tells you exactly how much material your beam removes on a given sheet, so you can set up finger joints, tabs and inlays that fit the first time instead of the third. All you need is a small offcut, a pair of digital calipers and a simple test file you can draw in a couple of minutes.

If you are not sure what kerf is or why it makes tabs loose, start with laser kerf explained. This article is the practical part: how to measure it.

What you need

An offcut of the exact material you plan to use, at least 130 × 40 mm. Same sheet, same thickness, same brand if possible.
Digital calipers. A ruler is not precise enough; you are measuring tenths of a millimetre.
The settings you will actually cut with: power, speed, passes, focus and air assist. Kerf changes with every one of them.
A drawing program or your laser software to draw a few rectangles.

Before you start, clean the lens, check focus, and measure the sheet thickness in three or four places. Write the thickness down. It matters as much as the kerf when you design joints.

Method 1: the strip test (most accurate)

This method spreads the kerf over several cuts, so small measurement errors shrink.

Draw the test

Draw a rectangle 100 mm wide and 20 mm tall.
Inside it, draw 9 vertical lines at 10 mm spacing, from the top edge to the bottom edge. You now have 10 strips, each 10 mm wide, sharing their edges.
Make sure there is only one line at each position. No overlapping duplicates, or some cuts will run twice and widen.
Put everything on your cut colour (for example red #FF0000) with a hairline stroke.

Cut and measure

Place the offcut flat and cut the test with your normal cut settings.
Leave the strips in place for a moment. Lift them out carefully, keeping them in order.
Stand the 10 strips on edge, squeeze them tightly together side by side, and measure the total width with calipers.
Work out the kerf: kerf = (100 − measured width) ÷ 10.

For example, if the 10 strips measure 98.5 mm together, the kerf is (100 − 98.5) ÷ 10 = 0.15 mm.

Why it works: each strip loses half a kerf on both sides, so each comes out one kerf narrower than 10 mm. Ten strips lose ten kerfs in total.

As a cross-check, put the strips back into the hole and push them all to one side. The gap left over should be about 11 times the kerf, since the hole also grew by one kerf. Measure that gap with calipers or feeler gauges if you have them.

Method 2: the square test (quick)

Good for a fast check when you change material.

Draw a 20 × 20 mm square.
Cut it out of the middle of the offcut so you have a square part and a square hole.
Measure the part across both directions. Kerf ≈ 20 − part size.
Measure the hole across both directions. Kerf ≈ hole size − 20.
Average the two results.

This is less accurate than the strip test because a single cut is close to the limit of what hobby calipers can read reliably. If the part and hole give very different numbers, or X and Y differ a lot, check your belts and focus.

Method 3: the fit test (what really matters)

Numbers are useful, but the real question is: does the joint feel right? A fit test answers it directly.

Draw a strip about 120 mm long with 7 slots in it. Make each slot as deep as you like (10 mm is fine) and vary the width: sheet thickness minus 0.3 mm, minus 0.2, minus 0.1, exact, plus 0.1, and so on, in steps of 0.05 or 0.1 mm.
Label each slot with a small scored or engraved number.
Cut the strip and a few small tabs from the same sheet.
Push a tab into each slot. Note which slot gives a firm press fit and which gives an easy slip fit.

The difference between the slot that fits and the nominal thickness tells you how much allowance your joints need. Many makers keep a labelled fit strip for each material on the wall above the machine.

How to do a laser kerf test that you can trust

A few details make the difference between a useful number and a misleading one:

Cut in the middle of the bed and the sheet, not on a warped corner.
Use your real settings. Testing at 80% power and then cutting at 65% gives the wrong answer.
Let parts cool before measuring. Acrylic in particular can be slightly warm and soft just after cutting.
Measure at the same height. Because of taper, the top face of a cut is often wider than the bottom. Measure near the middle of the thickness, or measure both faces and use the average.
Repeat once. If two runs agree to within about 0.02 mm, you are done.
Test both directions if your machine might cut slightly differently along X and Y.

Using your result

You now have a kerf value. Put it to work in one place only:

In a parametric design tool. Enter it as the kerf when you generate the file. The Digital Creation laser cut tool starts from sensible values for 3 mm material on a CO2 laser (0.15 mm for plywood, birch plywood and MDF; 0.12 mm for basswood, cast acrylic, cardboard and veg-tanned leather), and you can compare your measured number against these.
In LightBurn. Use the Kerf Offset setting on the cut layer. It is applied to each edge, so enter half your measured kerf.
In your own drawing. Offset outer shapes outward and holes inward by half the kerf.

Do not combine these. If a file already allows for kerf, adding a software offset as well will make joints too tight.

Keep a kerf log

Write down every result. A simple list on paper or in a notes app is fine. For each test, record:

Material and brand
Measured thickness
Machine and lens
Power, speed and passes
Measured kerf
Best slot for press fit and for slip fit

After a few weeks you will have a reference that saves you from testing every time. Repeat the test when you change lens, replace a tube, switch material supplier, or notice joints getting looser.

Common problems

Strips are uneven widths. The sheet moved, or the bed is not level. Pin the sheet down and recheck focus across the bed.

Strips are still attached. Not enough power for a clean cut. Adjust settings first; a test with incomplete cuts is not valid.

Results jump around between tests. The material varies, or measurements are being taken on burnt edges. Lightly brush off soot and measure firmly but without crushing.

Numbers look fine but joints are still loose. The sheet is thinner than the design assumed. Use the measured thickness, not the label thickness.

As always, never leave the laser running unattended, keep ventilation on, and keep a fire extinguisher close.

Once your kerf is known, the rest of the workflow is in how to make laser cut files. When you are ready to try it on a real project, generate a finger-jointed box with your measured value and see how the corners go together.

Frequently asked questions

How do I measure laser kerf?

Cut ten 10 mm strips that share their edges, squeeze them together, and measure the total. Subtract that from 100 mm and divide by 10 to get the kerf.

What tools do I need for a kerf test?

An offcut of your material, digital calipers, and a simple test file. Feeler gauges are a useful extra for measuring small gaps.

How often should I do a kerf test?

Each time you change material, thickness, settings, lens or tube, and whenever joints start to fit differently than before.

Should I use the full kerf or half the kerf?

The kerf is the full cut width. Most offset settings, including LightBurn's Kerf Offset, apply to each edge, so they use half of it.