What Size Biscuit for 3/4-Inch Wood? #20, Except on Narrow Parts

Use a #20 biscuit for most joints in 3/4-inch wood. It is the largest standard size, it gives the most glue area, and a 3/4-inch board has plenty of thickness to hold its 4 mm slot. Drop to a #10 or #0 only when the part is too narrow for the #20 slot to fit, such as a slim rail or a small frame member.

The part that most guides get wrong is the reason. Board thickness does not choose the biscuit size, because every standard biscuit is the same thickness. What decides the size is how much width and length the part has, and whether your joiner’s depth dial is set to match the biscuit you are using. This guide gives the real dimensions, the arithmetic for 3/4-inch stock, and a depth-setting mistake that leaves gaps in otherwise good joints.

The Short Answer for 3/4-Inch Wood

Here is the quick version. Use a #20 for edge-to-edge glue-ups and for wide parts such as cabinet sides and shelves, a #10 when the part is a bit too narrow for a #20, and a #0 or a face-frame (FF) biscuit for slim rails and small pieces. The rest of this page shows where those lines fall and why.

Biscuit Sizes Compared: #0, #10 and #20

Lamello invented the biscuit joiner and its sizes are the reference most brands follow. Their published dimensions, plus the cutting depths for each size from the Lamello manual as quoted by a woodworker on the Festool Owners Group forum, are below. Every size shares the same 4 mm (about 5/32-inch) thickness.

Size	Length x Width x Thickness (mm)	Inches (approx.)	Lamello cutting depth
#0	47 x 15 x 4	1-27/32 x 5/8 x 5/32	8.0 mm (5/16 in)
#10	53 x 19 x 4	2-1/8 x 3/4 x 5/32	10 mm (0.4 in)
#20	56 x 23 x 4	2-3/8 x 1 x 5/32	12.3 mm (0.48 in)

Two other sizes show up on 3/4-inch projects. Porter-Cable sells a face-frame (FF) biscuit that is 30 x 13 mm and is meant for face-frame parts 1-1/2 inches wide and up. Smaller “detail” biscuits such as the R1 through R3 sizes are only 2.4 mm thick and need a different, thinner cutter, so they are not interchangeable with the sizes above.

Why 3/4-Inch Thickness Does Not Pick the Size

A common claim is that 3/4-inch stock needs a #0 (or, in other places, a #10) because the wood is “thin.” The numbers say otherwise. Three-quarters of an inch is 19.05 mm. A biscuit slot is 4 mm wide, so if the slot is centered the wood on each side of it is (19.05 – 4) / 2, or about 7.5 mm. Lamello lists a minimum material thickness of 6 mm for edge cuts on its #0, #10 and #20 biscuits, and 7.5 mm clears that with room to spare.

Because the slot is the same 4 mm for all three sizes, the wall of wood beside it is the same for all three sizes too. A #20 does not weaken a 3/4-inch board any more than a #0 does. The one condition is that the slot stays centered in the thickness, so check that your fence is set for center and not for an offset cut.

What Actually Picks the Size: Part Width and Joint Type

The #20 slot is 56 mm (2-3/8 inches) long. Lamello quotes a fitting tolerance of plus or minus 1 mm, so the slot needs solid wood along its whole length with some margin at each end. That is why narrow parts push you toward a smaller biscuit. Use this as a working guide for 3/4-inch stock:

Situation in 3/4-inch stock	Biscuit to use	Reason
Edge-to-edge glue-up of wide boards (panels, tabletops, shelves)	#20	Longest slot and largest glue area, and a long board edge has room for it
Cabinet sides, tops and bottoms meeting at 90 degrees	#20	Wide parts give the slot room; you can double up on big parts
Rails or stiles about 2 to 3 inches wide	#10	A 53 mm slot leaves more margin at the ends than a 56 mm slot
Face-frame parts 1-1/2 inches wide and up	FF or #0	Porter-Cable specifies the FF size for that width
Small, narrow or delicate pieces	#0	Shortest slot at 47 mm, so it fits the least stock

The rows for #10 and #0 are practical rules of thumb, not manufacturer minimums, so dry-fit on scrap before you commit to a size. For the edge-to-edge case, remember that a long-grain glue joint is already strong. On a glue-up the biscuits mostly keep the boards aligned so the faces stay flush, and they add real strength where a joint is weak, such as a butt or mitre joint. We cover that difference in our guide on how strong biscuit joints really are.

For spacing, a common habit is a biscuit about every 6 to 8 inches along an edge, starting a couple of inches in from each end. On a 36-inch board edge with biscuits on 8-inch centers and 2 inches of clearance at each end, that works out to five biscuits. Treat that spacing as a guideline rather than a rule.

Set the Cutting Depth to Match the Biscuit

Biscuit joiners have a depth dial marked 0, 10, 20 and often other settings. This is the setting that most often ruins a joint when the size is right. Each biscuit sits half in one slot and half in the other, so the two slots together must add up to the biscuit width. Using the Lamello figures above:

• #20 biscuit, dial left on #0: two slots at 8.0 mm give 16 mm of room for a 23 mm biscuit. It is about 7 mm short, so the biscuit will not seat and the joint will not close.
• #0 biscuit, dial left on #20: two slots at 12.3 mm give 24.6 mm of room for a 15 mm biscuit. That leaves about 9.6 mm of empty slot, roughly 4.8 mm on each side, so the biscuit floats loose and gets little glue contact.

Both errors happen when you change biscuit size mid-project and forget the dial. Turn the dial every time you change size, and cut a test slot in scrap before the real parts.

Brands Are Not Identical

The sizes are named the same across brands but the dimensions drift. The Wikipedia table for Porter-Cable lists its #20 at 58 x 24 mm against Lamello 56 x 23 mm, and its #0 at 47 x 16 mm against 47 x 15 mm. A Trend pack listed on Amazon gives its #20 as 2-3/8 x 29/32 inch, which is about 60 x 23 mm. The WEN #20 pack in our pick above is listed at 56 x 23 x 4 mm.

#20 biscuit	Length x Width (mm)
Lamello	56 x 23
WEN (Amazon listing)	56 x 23
Porter-Cable (per Wikipedia)	58 x 24
Trend (Amazon listing, converted from inches)	about 60 x 23

A few millimeters matter because a biscuit slightly wider than your dial expects will not seat, while a slightly narrower one leaves a small void. A woodworker on the Festool Owners Group forum noted that Porter-Cable biscuits run a little wider than Lamello ones, so the depth setting may need a small adjustment. The simple fix is to buy one brand of biscuit for your joiner and test-fit before glue.

How to Biscuit-Join 3/4-Inch Boards

1. Dry-fit the parts. Lay the boards together the way they will be glued and pencil a line across the joint at each biscuit location.
2. Set the depth dial to the biscuit size. Match it to #0, #10 or #20 as in the section above.
3. Center the fence. On 3/4-inch stock you want the slot in the middle of the thickness, so the wall stays about 7.5 mm on both sides.
4. Clamp the workpiece and plunge. Hold the joiner face flat to the board, align its center mark with your pencil line, and plunge steadily. Keep your hands clear of the cutter, and wear eye and hearing protection.
5. Test on scrap. Cut a pair of slots in an offcut and check that a biscuit slides in with light hand pressure.
6. Glue and assemble. Use water-based wood glue. Lamello notes that its beech biscuits swell when wet, which tightens the joint as the glue cures, so assemble promptly once the glue is in the slots.
7. Clamp and wipe. Apply even clamp pressure, wipe squeeze-out, and leave the assembly clamped until the glue has set.

If you are choosing between biscuits and other joinery for a project, our comparison of seven ways to join wood covers the alternatives, and this page on biscuit thickness lists the dimensions in a different layout.

Troubleshooting Common Problems

• Biscuit will not go in: check the dial first. A biscuit too wide for the slot depth stops short, and swollen or damp biscuits will not seat either. Keep biscuits in a sealed container.
• Joint will not close: a #20 in slots cut at a smaller setting is the usual cause, as the arithmetic above shows.
• Biscuit is loose in the slot: the slot may be cut deeper than the biscuit needs, or the biscuit is a smaller size than the dial. Use fresh biscuits and match the setting.
• Faces sit out of flush: the two slots were cut with different fence heights or the joiner was tilted. Reference every cut from the same face of the board.
• Biscuit shows at the surface: the slot was too close to the edge or the part was too narrow for that size. Move to a #10, #0 or an FF biscuit.

Frequently Asked Questions

What Is the Standard Biscuit Size for 3/4 Wood?

A #20 biscuit (56 x 23 x 4 mm in Lamello sizing) is the standard choice for 3/4-inch boards joined edge to edge or in wide parts. Use a #10 or #0 when the part is too narrow for the #20 slot.

Does Wood Thickness Decide the Biscuit Size?

No. All standard biscuits are about 4 mm thick and cut a 4 mm slot. A centered slot in 3/4-inch stock leaves about 7.5 mm of wood on each side, above the 6 mm minimum Lamello lists, so thickness does not limit the size. Part width and slot length do.

Can I Use a #0 Biscuit in 3/4-Inch Wood?

Yes, it fits and works. It gives less glue area than a #20, so it suits narrow or small parts. Set the joiner depth dial to #0 first, because a #0 slot is only 8.0 mm deep in Lamello sizing.

What Happens If the Depth Dial Does Not Match the Biscuit?

The joint will not fit right. A #20 biscuit in slots cut at the #0 setting is about 7 mm too wide for the two slots and will not close. A #0 biscuit in #20 slots sits loose with about 9.6 mm of empty slot.

Conclusion

For 3/4-inch wood, reach for a #20 biscuit on edge joints and wide parts, and step down to a #10, #0 or FF biscuit for narrow parts. Thickness is not the deciding factor, since a centered 4 mm slot leaves about 7.5 mm of wood on each side of 3/4-inch stock. What deserves your attention is the depth dial, the width of the part, and staying with one brand of biscuit so the sizes match your joiner.