Biscuit Joint Layout Calculator

How it works

A biscuit joiner cuts a crescent-shaped slot with a 4 mm wide blade; a thin wooden biscuit glued into matching slots in both pieces aligns and joins them. The common sizes are #0, #10 and #20 (47, 53 and 56 mm long) and the small FF size for face frames.

The slot is centred on the stock when the fence holds the blade's centre at half the stock thickness. The wood left on each side of the slot is the thickness less 4 mm, halved.

Biscuits are spaced along the cut face: the first and last at the end margin, the rest evenly between them, using the fewest biscuits that keep every gap at or under the maximum spacing. For a miter the cut face is longer than the board is wide, by 1 ÷ cos of the cut angle (√2 for 45°). A joint shorter than two end margins gets one biscuit at its centre. The calculator also compares the biscuit's length with the cut face, and names the largest size that is no longer than it.

Formula

cut face      = joint length ÷ cos(cut angle)
span          = cut face − 2 × end margin
biscuits      = ceil(span ÷ maximum spacing) + 1     (1 when span ≤ 0)
spacing       = span ÷ (biscuits − 1)
centre k      = end margin + (k − 1) × spacing
fence height  = stock thickness ÷ 2
wood per side = (stock thickness − 4 mm) ÷ 2

Example

A 24 in edge joint in 3/4 in stock with #20 biscuits, 2 in end margins and at most 8 in between biscuits: the 20 in span needs 3 gaps, so 4 biscuits at 6.6667 in centres (2, 8.6667, 15.3333 and 22 in). The fence is set 3/8 in (0.375 in) to the slot centre, leaving 0.2963 in of wood on each side of the 4 mm slot. A #20 biscuit is 56 × 23 × 4 mm (2.20 × 0.91 in).

A 45° miter on a 2-1/2 in wide frame member has a 3.5355 in cut face (2.5 × √2). That is less than two 2 in margins, so it takes one biscuit at 1.7678 in; the largest size no longer than the face is #20.

Assumptions and limitations

• Biscuit dimensions are those listed for Lamello #0, #10 and #20 and Porter-Cable FF (Wikipedia, Biscuit joiner); other brands differ by a millimetre or two. The 4 mm slot width is the standard cutter.
• The FF slot is assumed to be cut with the standard 4 mm cutter; the source gives no FF thickness.
• The end margin and spacing are typical practice (about 2 in from each end, 6 to 12 in apart), not manufacturer requirements, and are editable.
• The slot is centred on the stock thickness. An offset slot, a double row of biscuits in thick stock and the joiner's own fence scale are not modelled.
• The fit check compares the biscuit's length with the cut face only. The slot a joiner cuts is somewhat longer than the biscuit, so a face only slightly longer than the biscuit can still show the slot at its ends.

Frequently asked questions