Biscuit Joint Calculator

Biscuit Joint Calculator for panel glue-ups, face frames, shelving and miters. It finds the largest biscuit that fits, spaces the slots evenly and warns before a slot cuts through.

Plan Slots with the Biscuit Joint Calculator

The Biscuit Joint Calculator turns a joint's length and stock thickness into a full slot layout. It picks the biscuit size, sets the plunge depth, spaces the slots and totals the biscuits for the whole project.

It handles edge-glued panels and tabletops, fixed shelves set into a cabinet side, and 45° miters on boxes and carcasses. Face frames work too, down to FF biscuits on 1-1/2 in rails.

What to Enter for Each Joint Type

Pick Edge or End, T-Joint or 45° Miter at the top. Then enter the actual stock thickness, not the nominal size. A 3/4 in plywood sheet often measures 23/32 in, and 4/4 hardwood planed S2S usually lands at 13/16 in.

Joint length is the edge being joined. On a face frame that is the rail width, and on a fixed shelf it is the shelf depth. For a panel, enter the number of boards, and the tool counts one glue line fewer. Results read in inches unless you switch to millimeters under Spacing and Units.

Joint Length Decides the Biscuit Size

Many size charts tie biscuit size to board thickness. On an edge joint that rule doesn't hold, because #0, #10 and #20 biscuits are all 4 mm thick and ride in the same 4 mm kerf. Thickness only changes how much wall is left on each side.

What limits size on an edge is length, since the blade cuts a slot longer than the biscuit. The Porter-Cable 557 manual says to use the largest biscuit the joint will take, and Auto does that with wood to spare at both ends.

Sizes and depths for #0, #10 and #20 are Lamello's published settings from the Lamello System brochure. FF is Porter-Cable's 13 × 30 mm face frame biscuit, cut with a separate 2 in blade. Its depth is the calculator's estimate, so test it on scrap.

Slot lengths assume a 4 in blade, or 2 in for FF. The last column adds 1/16 in of wood at each end, which is the calculator's own margin rather than a published rule.

Take a 1-1/2 in face frame rail. A #0 slot is 2.155 in long, so only FF fits, with about 0.06 in left at each end, and the tool marks it tight. A 2-1/2 in rail takes a #0 with 0.17 in to spare.

The Every Size on This Joint table runs all four sizes on your entries and shows why each fits or fails. When even FF is too long, dowels are the better choice, and the Dowel Spacing Calculator lays them out.

Why the Slot Is Longer Than the Biscuit

A biscuit joiner plunges a round blade straight into the edge. The slot you see at the surface is the chord of that circle at the plunge depth.

$$L_{slot}=2\sqrt{d(D-d)}$$

Here d is the plunge depth and D is the blade diameter. With a 4 in (101.6 mm) blade and the #20 depth of 12.3 mm, the slot comes out at 66.3 mm, or 2.610 in.

That is 0.405 in longer than a 56 mm biscuit, which the Side-to-Side Play row shows. The extra length lets you slide boards into line before the clamps go on. Lamello's 100 mm cutter cuts a #20 slot of about 65.7 mm, so the figure shifts slightly between machines.

Wall Each Side is the stock thickness minus the 4 mm kerf, split in half. On 3/4 in stock that leaves 0.296 in per side. Under 1/2 in thick the wall drops below 0.171 in, and the tool asks for a test slot first.

How the Biscuit Joint Calculator Spaces Slots

The two outer slots go at your end distance. The calculator then fits the fewest slots that keep every gap at or under your maximum spacing.

$$n=\lceil \frac{L-2e}{s_{max}}\rceil +1$$

L is the joint length, e is the end distance and s max is your spacing limit. The actual center spacing then shares the span equally between the slots.

$$s=\frac{L-2e}{n-1}$$

On the default 30 in joint with 2 in ends and an 8 in limit, the span is 26 in. That takes 5 biscuits at 6-1/2 in centers, and the Layout Marks table lists each center from 2 in out to 28 in.

End distance has a floor too. A slot's tip reaches half its length past the center mark, so the first center has to sit at least that far in, plus 1/16 in of wood.

$$e_{min}=\frac{L_{slot}}{2}+0.0625$$

For a #20 that works out to 1.367 in, so an entry of 1 in is moved out to that point with a note. Clear Wood at Ends is measured from the slot's tip, which is why it reads 0.695 in on the default layout.

Picking a Maximum Spacing

No standard sets biscuit spacing, so the 8 in default is a starting point. One member of the Fine Woodworking forum uses about 6 to 8 in on plywood carcasses and 10 to 12 in where biscuits only align a face frame.

A Sawmill Creek member who started at 4 to 6 in found his joints no straighter than at 8 to 12 in, just more expensive. On a panel that will be trimmed later, keep slots 2 to 3 in inside the final edge.

The Biscuits Total card multiplies the per-joint count by every glue line. Four boards make three glue lines, so the default comes to 15 biscuits and 30 slots, one slot in each board for every biscuit.

Face Slots in a T-Joint

On a fixed shelf, the shelf end takes an edge slot but the cabinet side takes a slot in its face. That face slot has to stop short of the back, so here panel thickness really does limit biscuit size.

The Skin Behind Face Slot row is panel thickness minus plunge depth. A 3/4 in side keeps 0.266 in behind a #20. A 1/2 in panel keeps only 0.016 in behind a #20 and 0.106 in behind a #10.

Only a #0 leaves more than 1/8 in in 1/2 in stock, at 0.185 in. That fits advice in a Fine Woodworking thread on 1/2 in plywood, where a member warns that larger sizes break out the far side.

The 1/8 in skin is the calculator's own safety margin. Lamello's wooden biscuit page lists 13 mm as the thinnest panel for a #20 surface cut, which leaves under 1 mm behind the slot.

Keeping a 45° Miter Slot Inside the Wood

A miter slot is cut square to the 45° face, so it plunges toward the outside show face. On 3/4 in stock that face is only 1.061 in across, and a centered #20 runs 0.023 in past the outside surface.

The fix is to move the slot toward the inside corner. Shift Toward Inside gives 0.278 in here, which leaves 0.174 in of wood over the biscuit. Members of the NC Woodworker forum describe the same blowout and the same cure.

A #10 on the same stock clears when centered, but only by 0.041 in, and a 0.241 in shift raises that to 0.211 in. Where no biscuit fits a thin miter, a spline joint reinforces the corner instead.

Strength, Swelling and Slot Placement

Do biscuits make an edge-glued joint stronger?

Not by any amount that matters on long grain. A glued edge joint between flat, well-jointed boards is already stronger than the wood beside it. One woodworker on Sawmill Creek who broke test panels saw the break land away from the glue line every time.

Biscuits earn their place where glue alone is weak, such as end grain, plywood and particleboard edges, and cross-grain cabinet joints. For a frame that carries real load, like a table apron, a mortise and tenon is the stronger joint.

What size biscuit works in 3/4 inch plywood?

For an edge or corner joint, a #20 fits any joint at least 2.735 in long, and 23/32 in plywood keeps 0.281 in of wall on each side. A face slot keeps 0.234 in of skin, but a 45° miter in the same sheet needs the slot moved 0.274 in toward the inside corner.

Why do biscuit joints leave dips after sanding?

Water from the glue swells the biscuit and the wood around it. If you sand the panel flat while that spot is still raised, it shrinks back as it dries and leaves a shallow, biscuit-shaped dip.

The biscuit itself grows very little. One woodworker on Woodworking Talk measured a Porter-Cable #20 at 0.156 in dry and 0.163 in after soaking in glue. The wet wood around the slot moves more, so let the joint dry a few days before final sanding.

Should the slot be centered on the board's thickness?

Centered is right when both faces show. When only one face shows, several woodworkers set the slot below center, nearer the underside, so any swelling stays away from the show face.

Either way, register the joiner fence on the same face of every board. Referencing one board from the top and the next from the bottom is a common reason slots fail to line up.

When should two biscuits go in one joint?

Thick stock has room for a second biscuit. The Porter-Cable 557 manual cuts the slots as usual, then cuts again with the alignment plate as a spacer, which widens each groove to take two biscuits side by side.

The calculator suggests this from 1-1/2 in stock up. Two per slot or a second row doubles the Biscuits Total count, so order biscuits to match.