What Is Kerf? Why Your Laser-Cut Joints Don't Fit

You measured everything. The box generator asked for 3 mm plywood, you fed it 3 mm plywood, and the finger joints still rattle in their slots like loose teeth. Or you cut a hole for a 6 mm dowel and the dowel dropped straight through without touching the sides.

Nothing is wrong with your file. You just met kerf.

The word is older than any of our machines. Kerf originally meant the slot a saw blade leaves in a board, and carpenters have been accounting for it for centuries. Your laser inherited the problem, just at a much smaller scale.

The Marker Line Problem

Here is the whole concept in one picture you can hold in your head.

Your design software draws with an infinitely sharp pencil. A line in an SVG has position but no width. Your machine, though, cuts with a marker. The beam or the bit removes a channel of real width, centered on your pencil line.

That center part matters. Half the channel falls on one side of your line, half on the other. So every cut edge in your finished piece sits about half a kerf away from where you drew it, on both sides, on every cut, everywhere in the design.

Draw a 20 mm square and cut around it. The channel eats half a kerf inward all the way around, so the square you pick up off the bed is a full kerf narrower than 20 mm. Meanwhile the square hole left behind in the sheet is a full kerf wider than 20 mm. Same line, two different results, depending on which side of the cut you keep.

That is the entire mystery. Everything else in this post is just measuring the channel and deciding what to do about it.

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How Big Is Kerf on Each Machine?

Kerf is not one number. It depends on the machine, and within one machine it shifts with material, thickness, focus, power, and speed. But the order of magnitude per process is worth knowing, because it tells you when to care.

Two things jump out of that table.

First, CNC kerf is enormous compared to laser kerf, but CNC workflows have dealt with it for so long that the fix is built in. Your CAM software asks whether to cut inside, outside, or on the line. That question is kerf compensation wearing a different name, and answering it correctly makes the bit width mostly disappear from your finished dimensions. Laser software, by contrast, usually cuts on the line by default and quietly hands you the problem. If you are weighing the two machines against each other, our laser cutter vs CNC router comparison goes deeper on differences like this.

Second, a vinyl cutter has next to no kerf worth thinking about on thin material. The blade parts the vinyl like a zipper instead of vaporizing a channel (its software still has a blade offset setting, but that corrects how the blade swivels, not removed material). If you came from a cutting machine background, this is why laser joinery surprised you.

Why Parts Run Small and Holes Run Big

Once you know the channel is centered on the line, the classic symptoms all explain themselves:

• Outer profiles come out undersized. The edge moves inward by half a kerf on every side, so any outside dimension shrinks by one full kerf.
• Holes and cutouts come out oversized. The edge moves outward by half a kerf all around, so the hole grows by one full kerf.
• Joints get hit twice. A finger joint is a narrow finger going into a slot. The finger loses a kerf, the slot gains a kerf, so the total slop between them is roughly two kerf widths. On a laser that can be 0.3 to 0.4 mm of wiggle, which is exactly the gap that makes a box joint rattle instead of press together.

This is also why compensation goes the direction it does. To land on your intended dimensions, the drawn hole needs to get smaller and the drawn part needs to get bigger, each by one kerf, so the material the machine eats brings them back to true size.

Before you touch anything, though, you need your actual number.

How to Measure Your Kerf

Do not borrow a kerf value from a forum post or a friend's machine. Kerf changes with the lens, the focus height, the material, its thickness, and your power and speed settings. The only number that means anything is one you measured on your machine, on the project material, with the project settings.

The test takes a few minutes and a pair of calipers. Cheap digital calipers are fine.

1. Draw a simple square of a known size, say 20 mm, in your design software.
2. Cut it from the same sheet you will use for the real project, with the exact settings you will use for the real cuts.
3. Measure the square you cut out, on both axes. It will read under 20 mm.
4. Measure the hole it left in the sheet. It will read over 20 mm.
5. Your kerf is the piece's shortfall (drawn size minus measured piece), and you can sanity-check it against the hole's excess (measured hole minus drawn size). The two should roughly agree.

For example, if your drawn 20 mm square measures 19.8 mm, your kerf is 0.2 mm, which means 0.1 mm per side. That per-side number, half the kerf, is what most offset tools want.

Write the result down. Kerf per material and thickness is one of those numbers worth keeping on a sticky note near the machine, because it barely changes until you change the lens, the material, or the settings.

Kerf Compensation: Four Ways to Fix the Fit

You have a number. Now you apply it, once, in exactly one place.

1. Let the generator do it. Box and joint generators almost always have a kerf field. Enter your measured kerf and the generator adjusts every finger and slot for you. For joinery, this is the easiest path by far.
2. Offset paths in your design software. Most vector editors have an offset or outset function. Grow outer profiles by half a kerf, shrink holes by half a kerf. Tedious on complex designs, but it works everywhere.
3. Use your laser software's kerf offset setting. Many cutting programs can apply the offset at send time, which keeps your design file clean and at true dimensions.
4. On the CNC, answer the toolpath question honestly. Inside for holes, outside for profiles, and the CAM software handles the bit diameter. Cutting on the line only makes sense when the geometry was already compensated somewhere else.

Once the design is compensated, the usual file logistics apply. If your machine's software wants DXF instead of SVG, File Converter does the conversion free, with no credits, and keeps your geometry intact. If you are unsure which format your workflow should be built around in the first place, our DXF vs SVG breakdown settles it.

When Kerf Matters and When to Ignore It

Here is the part that saves you from overthinking: most cuts do not need kerf compensation at all.

Compensate when parts have to mate:

• Finger joints and box joints
• Tabs and slots
• Press-fit parts, dowel holes, bearing seats
• Inlays, where the piece and the pocket move in opposite directions and the errors stack
• Gears, hinges, anything mechanical

Ignore it when nothing touches the edge:

• Engraving, which never cuts through and has no mating edge at all
• Decorative silhouettes, ornaments, and wall art
• Coasters, cake toppers, signs
• Stencils, unless the stencil is for something dimension-critical

The rule of thumb: if the part never has to fit another part, a tenth of a millimeter changes nothing anyone will notice, and your time is better spent elsewhere. Trace a photo or a logo into a clean SVG with MonoTrace, free with no credits, cut it as wall art, and kerf will never enter the conversation. It only becomes real the day two pieces have to hold each other.

And either way, kerf is just one item on the pre-flight list. Our guide to preparing files for laser cutting covers the rest of what to check before you press start.

Frequently Asked Questions

Is kerf the same for every material on the same laser?

No. Different materials burn away differently at the same settings, and thickness matters too. Measure per material and thickness, then reuse the number until something in the setup changes.

Why are my laser-cut edges slightly slanted?

The beam is focused to a narrow waist and spreads above and below it, so the cut walls can come out slightly tapered rather than perfectly vertical. How much, and in which direction, depends on your focus height and the material thickness. On thin material it is barely noticeable. On thick material it can affect how joints seat, which is one more reason to test-fit on the actual stock.

Can I just use the kerf number someone posted for my machine model?

Use it as a starting guess, not a final answer. Lens condition, focus, material batch, and settings all shift the real value. The quick square test tells you the truth about your machine today.

Measure Once, Fit Forever

Kerf is not a defect and your machine is not broken. It is just the width of the cut, and every cutting process ever invented has had one. The makers whose boxes snap together are not running better lasers. They measured a square once, wrote down a number, and typed it into the kerf field.

So next time the fingers rattle: cut the test square, do the subtraction, and recut. And when your compensated file needs to become a DXF for the machine, File Converter handles it free. For everything decorative in between, MonoTrace turns images into cut-ready SVGs, also free, no kerf math required.