Laser kerf: what it is and how to measure it

By meshcast.app · Updated November 2026

This guide was drafted with AI assistance.

What kerf is

Kerf is the width of material the laser removes as it cuts. The beam is not infinitely thin: it burns, melts or vaporises a narrow channel, and everything inside that channel is gone. Cut a 50 mm square and the part ends up slightly smaller than 50 mm, while the hole it leaves behind is slightly bigger.

For a single part you will never notice. For anything that has to fit, such as stacked layers, slot-together boxes, inlays or a name that sits in a backing plate, kerf is the difference between a snug joint and a wobbly one.

Typical kerf ranges

Kerf depends on the machine, the lens focus, the material, its thickness and the speed and power you cut with, so there is no single correct number. As a broad guide, treat these as typical starting expectations and then measure your own:

Two things catch people out. First, kerf grows with thickness: a deeper cut lets the beam spread and scorch, so 6 mm plywood usually cuts wider than 3 mm. Second, kerf is not constant across materials. Acrylic, wood and cardboard all react differently, so a value measured on one is only a starting point for another.

How to measure your kerf

Do not trust a number from a forum. Measure it on your own machine, with the material you intend to cut. You need digital calipers, which are cheap and accurate to 0.01 mm.

Method 1: cut a square and measure it

1. Draw a square of exactly 20 mm with no kerf compensation.
2. Cut it in the material and settings you plan to use.
3. Measure the finished square with calipers on two opposite sides. If it measures 19.8 mm, the kerf is about 0.2 mm (20 minus 19.8).

Measure more than once, in different spots of the bed, because focus and beam shape are rarely identical everywhere.

Method 2: the comb test

Cut a row of ten squares, each exactly 10 mm wide, drawn edge to edge in one line with no gaps. After cutting, push them together and measure the full row. If ten squares span 98 mm instead of 100 mm, you lost 2 mm across ten cuts, so the kerf is about 0.2 mm. Averaging over ten cuts reduces measuring error nicely.

Method 3: the fit test (tab and slot)

This one answers the question you actually care about. Draw a row of slots in a test plate, from a little narrower than your material thickness to a little wider, in steps of 0.05 mm. Cut matching tabs from the same sheet. Try each tab in each slot and write down which pairing is snug. That slot width is your working value, whatever the calipers say.

Press fit, snug fit and loose fit

Once you know the kerf, you decide how tight the joint should be. Material thickness varies too: plywood sold as 3 mm often measures a little under or over, so measure the actual sheet.

• Press fit: tab slightly wider than the slot, so assembly takes a mallet or a firm squeeze. Good for permanent joints in wood, risky in acrylic, which can crack under stress.
• Snug fit: the tab slides in with light pressure and stays put. This is the usual target for boxes you will glue.
• Loose fit: a visible gap. Fine for parts that must move, like hinges or a lid that lifts off.

Wood tolerates a tighter fit than acrylic and swells slightly with humidity, so leave a touch more room if the piece will live somewhere damp.

Kerf compensation: offsetting the outline

You can either live with the error or correct it in the design. Compensation moves each outline by half the kerf, because the beam centre travels along the line and half of its width falls on each side.

• An outside shape (a part you keep) is drawn half a kerf larger, so after cutting it lands on its nominal size.
• An inside shape (a hole or slot) is drawn half a kerf smaller, so the opening ends up the right size.

Many laser programs have a kerf or offset setting, but it is easy to apply it twice or in the wrong direction. Decide on one place to do it.

How Meshcast tools apply kerf

The laser STL slicer has a kerf compensation setting. It offsets every layer outline by half the kerf you enter, so stacked layers keep their intended size and interlocking features fit. Enter the value you measured with the tests above, not a guess. Leave it at zero if you already compensate in your laser software, otherwise the correction is applied twice.

The name tools, the cake topper, ornament and name sign makers, produce single flat parts, so kerf rarely matters except where a name is glued into a backing. Every export is SVG in millimetres (red #ff0000 for cut, blue #0000ff for engrave) or DXF with CUT and ENGRAVE layers. Browse the whole set on the laser hub. Designing and previewing are free, the first download is free, and further downloads use a free Meshcast account with a weekly allowance.

Common mistakes

• Measuring once. Take several readings and average them.
• Forgetting the material thickness. The slot has to match the real sheet, not the label.
• Mixing materials. Birch plywood and acrylic need their own numbers.
• Double compensation. Offset in the design or in the laser software, never both.
• Ignoring the burn angle. Thick material often cuts slightly tapered, wider on top. Measure the face that will be visible or glued.

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