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Laser Kerf Calculator: slots, tabs and test files that come out the size you meant

Every cut line eats a thin strip of material, so parts come out smaller than drawn and holes come out bigger. Measure how much your laser takes, then get the numbers to draw for a press fit, a glue fit or a slip fit. If you don't know your kerf yet, download a test file and find out in one cut.

Laser and material

Fills in a rough first guess for the kerf. It is a starting point for a test cut, not a measurement. Tab 2 turns a test cut into your real number.

Sheet thickness, measured

Kerf

Measure the sheet with calipers. "3 mm" plywood is often 2.7 to 3.2 mm, and a slot drawn for the label instead of the sheet is the most common reason a box won't go together.

How should the joint feel?

The number is the clearance: finished slot minus finished tab. Negative squeezes, positive leaves room. Wood forgives a press fit; acrylic cracks.

Width the tab and slot share

For a finger joint this is the finger width; for an inlay, the width of the piece.

Bolt, dowel or magnet diameter

For a hole that takes something you didn't laser-cut. Measure it: a "1/4 inch" dowel is often a little under.

Which part can you redraw?

"Only the slot" is for a tab on a part you bought or downloaded: the tab stays as drawn and the slot is sized around what it will really be. "Only the tab or inlay" is the other way around, for a pocket or slot already cut.

Draw it atComes out at

Tab ——

Slot ——

Slot for the sheet ——

Hole for a bolt or dowel ——

Or keep the drawing and offset every cut by—

Selling what you cut? Price the job in KraftyQuote →

The Math

Where every number above comes from

One rule does all of it: a drawn outline comes out a kerf smaller across, half a kerf off each cut edge, and a drawn hole comes out a kerf bigger. Here it is with your current numbers.

Tab, drawn—
Slot, drawn—
Finished slot − finished tab—
Slot for the sheet, drawn—
Hole for a bolt or dowel, drawn—
Kerf from strips(N × s − M) ÷ N
Kerf from the gapG ÷ (N + 1)

Both parts give up half the clearance. The tab is drawn a kerf wider than the shared width, less half the clearance; the slot a kerf narrower, plus half. Cut, the tab loses its kerf and the slot gains one, and what's left between them is exactly the clearance you asked for.

The sheet's own faces don't move. A slot that takes the sheet itself, a box side or a T-joint, only has the slot's cut edges to compensate. All of the clearance goes on the slot: thickness, minus kerf, plus clearance.

The gap counts one cut more than the strips. Ten strips in a frame are opened by eleven cut lines, because the frame's two inside edges are cuts too. Divide the gap by ten and the kerf reads ten percent high. Many kerf calculators get this one wrong.

In Your Software

Where the number goes

You can redraw the parts at the sizes above, or keep the drawing at its nominal size and let the software move the cut. The live value is the offset per cut edge for your current fit.

Software	Where	What to enter
LightBurn	Cut Settings Editor → Kerf Offset (closed shapes only)	The offset per edge, —. Positive grows outer shapes; LightBurn turns the other way for holes inside them on the same layer. LightBurn's own kerf test checks the result on your machine.
Illustrator	Object → Path → Offset Path	— on outer shapes, and the same amount negative on holes and slots.
Inkscape	Path → Path Effects → Offset	— outward on outer shapes, inward on holes. Set the effect's unit to match.
KraftyNest	Kerf Width (in)	The full kerf in inches, —. KraftyNest uses it to keep neighboring parts far enough apart that their cuts never overlap.

Offset a part or a slot, never both on top of a redrawn size, or the kerf gets compensated twice and the joint comes out a kerf loose. Software menus move between versions; the numbers don't.

Common Questions

What makers ask about kerf

Why is my laser-cut part smaller than I drew it?

The beam has width. The line in your file is the center of the beam, so half the beam's width comes off each side of every outline and gets added to each side of every hole. A 0.15 mm kerf turns a 10 mm square into a 9.85 mm square and a 10 mm hole into a 10.15 mm hole, which is exactly enough to make a joint rattle.

Is kerf the same on every material?

No. It changes with the material, its thickness, the lens, the focus, the power and the speed. Measure it once for each material and setting you use regularly and keep the numbers. On thick stock the cut is also slightly wider at the top than the bottom, so a joint can be tight on one face and loose on the other; cut a test and fit it the way the real part will go together.

Should I enter the full kerf or half in LightBurn?

The Kerf Offset moves the path of the beam, and each cut edge needs to move half a kerf to come back to the drawn size. So half, adjusted for your fit, which is the offset-per-edge figure above. LightBurn's documentation doesn't spell this out, so the honest answer is to cut its kerf test once with the value and check.

Why measure ten strips instead of one?

Calipers read to about 0.02 mm, and a kerf is often 0.1 to 0.2 mm, so on a single strip the error is a big share of the answer. Push ten together and the kerf is multiplied by ten while the caliper's error stays the same.

Will this fix a downloaded file that comes out loose?

Often. Most downloaded box and puzzle files are drawn for one particular laser's kerf. Use "Only the slot" mode to size slots around the tabs as they'll really come out on your machine, or apply the offset in your software to the whole file. Check the sheet thickness first: a file drawn for 3.0 mm will never fit a 2.8 mm sheet, whatever the kerf.

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