Every laser cut removes a strip of material, the kerf, and every part comes out smaller than drawn by half the kerf on each side. For decorative work nobody notices. For press-fit joints, inlays, and mating parts, that missing material is the entire difference between a joint that snaps together and one that falls apart. Kerf compensation, offsetting the vector path outward or inward by half the kerf width, is how you design for the beam instead of pretending it has zero width.
Measuring your actual kerf
Kerf width depends on the material, thickness, lens, focus, speed, and power, so there is no universal number to look up. Measure it on your machine with your settings. The reliable method: cut a test square of known drawn dimension, measure the resulting part with calipers, and divide the difference by two. That gives the per-side kerf for those exact conditions.
Cut the test in the same material and thickness as the real job, at the same speed and power, because kerf changes with all of them. Thicker material and slower speeds generally widen the kerf; tight focus narrows it. Keep a small log of measured kerfs per material and thickness. It takes minutes to build and pays off on every precision job afterward.
Applying offsets in the design
Compensation is a geometric offset of the cut path. For a part that must finish at its drawn size, offset the path outward by half the measured kerf, so the beam's center runs outside the intended edge and the removed strip falls in the waste. For a hole that must finish at its drawn size, offset inward by half the kerf. Most vector software and laser control programs include an offset or kerf-compensation function that does this automatically along the whole path, including around corners.
Corners deserve attention. A simple offset rounds inside corners by the beam radius no matter what you do, so design inside corners with relief or accept the radius. Outside corners offset cleanly. For finger joints and tabs, compensate the tab and the slot in opposite directions so the fit lands where you designed it, then tune with a test joint before cutting the full sheet.
Press fits and the test joint habit
Press-fit design lives or dies on kerf compensation plus a deliberate interference amount. After compensating for the measured kerf, add a small intentional overlap, often a few hundredths of a millimeter per side for laser-cut plywood, so the joint grips. Too little and the joint is loose; too much and assembly splits the material. The right value depends on the material's compressibility and the joint geometry.
Always cut a single test joint first. A small coupon with one tab and one slot, cut with the exact job settings, tells you more than any calculation. Adjust the interference in the design based on how the test fits: snug but assemblable by hand is the target for most plywood joinery. Change material batch, and re-test; plywood thickness and density vary enough between sheets to shift the fit.
When compensation matters less
Not every job needs it. Engraving removes no through-material and needs no compensation. Decorative cutouts where nothing mates can skip it. Parts cut deliberately oversize for later trimming do not need it either. Apply compensation where dimensions mate or where the drawing dimension is a functional requirement, and skip it where it adds work without benefit.
One subtle case: nested parts sharing cut lines. When two parts share a single cut line, the kerf comes out of both parts' dimensions, and offsetting both paths can double-compensate. In shared-line nesting, compensate once for the pair, or better, avoid shared lines on precision edges and give each part its own cut path with proper offsets.
Kerf versus beam width thinking
It helps to think of the laser not as a knife but as a milling cutter of a certain diameter. Just as a CNC operator accounts for cutter diameter in toolpaths, the laser operator accounts for kerf. The beam has a width, the width removes material, and the design must place the beam's centerline accordingly. Once that mental model clicks, compensation stops feeling like fudge factor and becomes ordinary toolpath practice.
Re-measure periodically. Kerf drifts as lenses age, mirrors drift out of alignment, and focus shifts. A kerf log with dates lets you spot drift before it ruins a precision run, and a quick test square is cheap insurance at the start of any critical job.
Kerf compensation is the habit that separates laser operators who cut parts from those who cut parts that fit. Measure the kerf under real conditions, offset paths by half of it in the correct direction, prove press fits with a test joint, and re-check as the machine ages. The beam has width; design like it.
Related Guides
- Corner Deceleration Smoothing in Vector Kerf Profiles: A laser cutting a vector corner has a physics problem: the motion system must slow down to change direction, but the beam keeps delivering full power
- Corner Radius Compensation for High Speed Laser Paths: At high cutting speeds, a laser cannot turn a sharp corner without consequence: the motion system must decelerate into the corner and accelerate out
- Lead-In and Lead-Out Geometry in Thin Sheet Cuts: Where the laser pierces thin sheet matters as much as where it cuts. A pierce is violent: the beam dwells and the entry point gets more heat than any
- Node Optimization Rules for Laser Cutting Speeds: A laser cutter's speed is not just the speed setting in the software; it is also the geometry of the vector file. Every node in a path is a point