1. Your joint
The joint edge is the full edge of the panel that carries the fingers. In the box generator that edge is the inner size plus two material thicknesses, so an inner width of 94 mm in 3 mm sheet gives a 100 mm edge. Use a dot or a comma for decimals.
2. Fingers on this edge
- Finger count
- -
- Finger pitch
- -
- Tab width to draw
- -
- Slot width to draw
- -
Results are in millimetres, with inches next to them when the form is in inches. The target finger width is 3 times the material thickness, the default of the box generator.
Test-cut strip
Three kerf settings, 0.05 mm apart, around the kerf you typed. Draw one tab and one slot at each setting on a scrap of the same sheet, cut them and keep the setting that fits best. This page gives the widths to draw; it does not draw the strip.
| Kerf setting | Tab drawn | Slot drawn | Slot minus tab once cut |
|---|---|---|---|
| - | - | - | - |
| - | - | - | - |
| - | - | - | - |
The last column assumes the beam removes exactly the kerf you typed: a positive value is a gap, a negative value is an overlap the material has to absorb. It is arithmetic, not a measured fit.
What kerf compensation does
Kerf is the width of material the beam burns away along a cut line. The beam follows the centre of the drawn line and removes half the kerf on each side, so a tab drawn 9 mm wide comes out one kerf narrower and a slot drawn 9 mm wide comes out one kerf wider. Left alone, every joint is loose by two kerfs. See laser cutting on Wikipedia.
Compensation moves the outline outwards by half the kerf on every cut side. The tab is drawn one kerf wider and the slot one kerf narrower, and the cut tab and the cut slot both come out at the pitch.
The formulas
- The edge is split into an odd number of equal segments, three or more, as close as possible to the target finger width. Where the edge is long enough a segment is at least 1.5 times the material thickness, and it is always wider than the material.
- pitch = edge length / finger count
- tab drawn = pitch + kerf
- slot drawn = pitch − kerf
- One panel keeps the even segments as tabs, both corners included. Its neighbour keeps the odd ones.
Example: a 100 mm edge in 3 mm sheet with 0.15 mm kerf gets 11 segments of 9.091 mm. The tab is drawn 9.241 mm wide and the slot 8.941 mm wide.
How to use the numbers
- Measure the kerf first: cut a 20 × 20 mm square and measure it with calipers. If it reads 19.85 mm, the kerf is 0.15 mm. The kerf guide has the steps.
- Measure the sheet too: nominal 3 mm sheet is often not 3.00 mm.
- Type 0 as kerf to see the joint without compensation: tab, slot and pitch are then the same number.
- A kerf setting a little above the measured one gives a tighter joint, a little below gives a looser one. The test-cut strip table shows both.
What is checked, and how
This page calls the same script as the box generator for the finger count, so both give the same numbers for the same edge. An automated fixtures script compares the calculator with the outlines the generator draws: for a 100 mm edge in 3 mm sheet with 0.15 mm kerf it measures the tabs and the slots on the bottom panel and requires the same widths. It does not check how the parts fit once cut: that depends on your machine and your material, and nobody has cut a part from these numbers.
Everything is computed in your browser. This page makes no request after it has loaded and stores nothing.
The full box and tray generator
This calculator covers one edge. The box and tray generator draws every panel of a finger-jointed box or tray with these formulas, with an optional lid and loose dividers, and packs the parts on your sheet. Its preview, part list and sheet size are free, as is this calculator.
Read this before you buy or cut
- Dimensions. You enter the inner width, depth and height. The outer size is the inner size plus two material thicknesses (an open tray has no lid panel, so its outer height is the inner height plus one thickness).
- Not test-cut. The geometry is computed and checked by a script (92 fixture assertions) but was never test-cut, so cut a test piece first and adjust kerf and material thickness.
- Made by AI agents. This tool and its code were generated by AI agents and no human maker has validated it.
- Price: $9 USD one-time.
- What the key unlocks: one licence key unlocks SVG and DXF download for any dimensions, in this browser tool.
- Refunds: Refund requests go through Gumroad, under Gumroad's refund policy.
- Free without a key. This calculator, and in the generator the form, the preview, the part list and the sheet size.