My laser-cut parts come out the wrong size or the joints do not fit. How do I use kerf compensation?

Why it happens

A laser removes a strip of material as wide as its beam. That width is the kerf. If the beam runs down the center of your line, half the kerf comes off the part and half comes off the waste, so outside shapes end up small and holes end up large.

Pick the right strategy on Laser Cut - Fill

1. Cut Outside / Right: the path is offset outward by the tool's kerf, so the finished part matches the drawn size. Use it for parts you keep.
2. Cut Inside / Left: the path is offset inward, so the hole or slot matches the drawn size. Use it for holes, slots and sockets.
3. Cut On: the beam center follows the vector with no compensation. Use it for score lines and decorative lines, not for parts that must fit.
4. Allowance: an extra offset for Cut Outside and Cut Inside that lets you ease or tighten a fit without editing the kerf in the tool. It is disabled for Cut On and Hatch Fill.

Measure your real kerf

The compensation is only as good as the kerf value stored in the laser tool. A published method that needs only a ruler:

1. Cut 10 identical squares with no compensation (Cut On).
2. Line them up edge to edge and measure the total length.
3. Kerf width = (10 x drawn size, minus the measured length) divided by 10.

The same guide notes that kerf changes with focus accuracy, speed, material and air assist (air assist narrows it), so measure on the job material at the job settings.

Tune the fit

• Enter the measured kerf in the laser tool, cut one tab and one slot, and test the fit.
• Adjust with Allowance in small steps and recut the test pair. The official documentation does not state which sign loosens a fit, so confirm the direction on scrap before a full sheet.
• A public Onefinity forum guide for a J Tech head adjusts kerf by eye for engraving fills: raise it if lines overlap, lower it if gaps show between lines.

Pro tips

• Kerf from one material does not carry to another. Store a laser tool per material.
• For slot-together plywood, thickness variation matters as much as kerf. See the entry on slot-together parts and thickness variation.