Dowel Joint Layout Calculator

How it works

Dowel size is usually chosen as a share of the stock thickness, typically about a third to a half, so that enough wood is left on each side of the hole. The calculator turns that range into inches and lists the standard sizes (1/4, 5/16, 3/8 and 1/2 in) that fall inside it, picking out the one nearest the middle.

Each piece is drilled to half the dowel length plus a small glue clearance, so the dowel does not bottom out and squeezed-out glue has somewhere to go.

Along the joint, the first and last dowels sit at the end margin and the rest are spread evenly between them, using the fewest dowels that keep every gap at or under the maximum spacing. A joint no longer than two end margins gets a single dowel at its centre. When the centres come out no farther apart than one dowel diameter, a note says the holes overlap.

Formula

diameter range = thickness × smallest % ÷ 100  to  thickness × largest % ÷ 100
hole depth     = dowel length ÷ 2 + glue clearance   (each piece)
span           = joint length − 2 × end margin
dowels         = ceil(span ÷ maximum spacing) + 1     (1 when span ≤ 0)
spacing        = span ÷ (dowels − 1)
centre k       = end margin + (k − 1) × spacing

Example

For 3/4 in stock, 33.3% to 50% of the thickness is 0.24975 to 0.375 in, which takes in 1/4, 5/16 and 3/8 in dowels; 5/16 in is nearest the middle. A 2 in dowel with 1/16 in clearance needs a 1.0625 in (1-1/16 in) hole in each piece.

Along a 24 in edge with 2 in end margins and at most 8 in between dowels, the 20 in span needs 3 gaps, so 4 dowels at 6.6667 in centres: 2, 8.6667, 15.3333 and 22 in from the end.

Assumptions and limitations

• The one-third to one-half diameter range, the 1/16 in glue clearance, the 1 to 2 in end margin and the 6 to 10 in spacing are typical dowel-joint guidance, not standards; all are editable.
• Only 1/4, 5/16, 3/8 and 1/2 in dowels are considered. Metric dowels (6, 8, 10 mm) are not listed.
• Dowels are centred on the thickness of the stock and spaced evenly. Joints that need extra dowels near the ends, or a dowel pattern dictated by a jig, are not modelled.
• The hole depth is the same in both pieces, and the calculator does not check that a hole of that depth fits within the width of either piece.

Frequently asked questions