Dowel and biscuit sizing
Both are loose tenons and both are sized by the stock — but by different things. A dowel comes off the thickness; a biscuit comes off how much room there is along the joint.
A corner joint is limited by how wide the narrower piece is
= 0.75"
6–10in is the usual range along an edge joint
Use
Dowel
1/4"
1 1/2" pin, bore 13/16" each side
Biscuit
#20
56 × 23mm, 0.3" either side of the slot.
How many
4
Centre to centre
6 11/16"
First and last, in from the ends
2"
Where to buy this
- 4 × 1/4" × 1 1/2" dowel pinsHome DepotLowe'sAmazon
- #20 biscuitsHome DepotLowe'sAmazon
Fluted dowel pins rather than plain dowel rod — the flutes give trapped glue somewhere to go, and a plain rod cut to length can hydraulic the joint open.
Plain search links, nothing sponsored. Prices and stock are the shop's, and a local lumberyard will usually beat both on solid stock.
Which biscuits this joint takes
- #0 — 47 × 15mm47 × 15mm, 0.3" either side of the slot.
- fits
- #10 — 53 × 19mm53 × 19mm, 0.3" either side of the slot.
- fits
- #20 — 56 × 23mm56 × 23mm, 0.3" either side of the slot.
- fits
The slot is a fixed 4mm whichever biscuit goes in it, so thickness rules biscuits in or out altogether and it is the room along the joint that picks the size. Minimum widths are the makers' published figures: 2 1/8" for a #0, 2 3/8" for a #10 and 2 3/4" for a #20.
Where the dowel size comes from
About a third of the stock thickness — the same rule of thirds a mortise and tenon works to, and for the same reason: a pin that size leaves enough wood either side of the hole that the hole is not the weak point. Half is the ceiling. A third of 3/4" is 1/4".
Length is really an observation about the shelf rather than a rule: pins are sold 1/4" × 1 1/2", 5/16" × 1 1/2" and 3/8" × 2", each about five times its own diameter. Each hole is bored a little past half, so the dowel cannot bottom out in both at once and hold the joint open on trapped glue.
What these are actually for
On an edge joint, alignment — not strength. A well-made edge-to-edge glue joint in long grain is already stronger than the wood around it, so there is nothing for a biscuit or a dowel to add. Keeping a wide panel flat while you clamp it is a real job and worth doing well, but do not skip jointing the edges properly on the theory that biscuits will hold it.
About dowels and biscuits
- What size dowel for 3/4" stock?
- 1/4", and a 1 1/2" pin. The diameter is the same rule of thirds a mortise and tenon works to — about a third of the thickness, half at the most — because that leaves enough wood either side of the hole that the hole is not the weak point. The length is really an observation about the shelf: dowel pins are sold 1/4" × 1 1/2", 5/16" × 1 1/2" and 3/8" × 2", and each is about five times its own diameter.
- Which biscuit should I use?
- The biggest one the piece is wide enough for. The slot is a fixed 4mm whichever biscuit goes in it, so the biscuit's size is limited by room along the joint rather than by thickness: #0 needs a 2 1/8" wide piece, #10 needs 2 3/8" and #20 needs 2 3/4". A long edge joint takes #20s; a 2 1/2" rail into a stile takes #10s.
- Can I use biscuits in 1/2" material?
- Just about — 1/2" is the practical floor, and it leaves 11/64" of wood either side of the slot. Below that, don't: 3/8" stock leaves about 7/64", which you can feel through the face and which the cutter will wander out of. Thin stock wants dowels, and this page offers them rather than recommending a biscuit it doesn't believe in.
- Do biscuits or dowels make an edge joint stronger?
- Not meaningfully, and it is worth being straight about it. A well-made edge-to-edge glue joint in long grain is already stronger than the wood around it, so there is nothing for a biscuit to add — what it adds is alignment, which is a real job and worth doing well on a wide panel. Where they genuinely add strength is a joint with end grain or short grain in it: a mitred corner, or a rail into a stile, where glue alone has little to hold.
You might also need
- Mortise and tenon sizingTenon thickness and shoulder by the rule of thirds, and the length a blind or through mortise allows.
- Clamping pressureHow many clamps a glue-up takes to reach the pressure the glue maker asks for, and how far apart.
- Panel glue-up calculatorHow many boards it takes to glue up a target panel width, and how much wider to glue up so the rip lands clean.