How do I drill dowel holes that line up?

Why dowel joints go wrong in three dimensions at once, and the two methods — a self-centring jig, or dowel centre points — that remove the guesswork.

Difficulty: intermediate
Time: 30 min
Read: 4 min

Short answer

Do not measure both sets of holes. Either use a self-centring dowelling jig registered off the same face of both parts, or drill the first part, drop dowel centre points into the holes and press the second part against them so they mark their own positions. Drill dead square with a depth stop, and always work from the marked face side.

A dowel joint has no tolerance. Two holes that are a millimetre apart in position, or a degree out of square, produce a joint that will not close or that closes crooked. The whole craft of dowelling is in never relying on two independent measurements agreeing.

What you'll need

A self-centring dowelling jig, or dowel centre points
Fluted dowels in a size around a third of the timber thickness
A drill with a depth stop or tape marker
Clamps

Step by step

Mark a face side on every part and reference everything from it.A jig registered off the face on one part and the back on the other puts the holes at different depths from the show surface, and the joint is out by the difference in thickness.
Clamp the parts together in their final position and tick across the joint.One pencil line crossing both pieces per dowel. The tick, not a measurement, is what both holes are aligned to.
Choose a dowel about one third of the timber thickness.Bigger removes too much material and weakens the parts; smaller does not carry the load. Two dowels per joint resist rotation; one does not.
Set the drilling depth before you start.Half the dowel length in each part, plus a couple of millimetres so the dowel cannot bottom out. Mark the bit with tape or fit a depth stop.
Method one — use a self-centring jig.Clamp it over the tick with the jig's reference face against the marked face of the workpiece. Drill through the guide bush. Repeat on the mating part from its face side.
Method two — use dowel centre points.Drill the holes in one part only. Drop a centre point into each hole, offer the second part up in exactly the right position, and press or tap it. Each point pricks the mating position perfectly, and you drill on the pricks.
Keep the drill square in both planes.This is the failure most people do not notice. Use the jig if you have one; otherwise stand a square beside the bit and sight it, or use a drill guide block. A hole a couple of degrees off pushes the joint open at one end.
Clear the holes and dry fit before glue.Blow out the chips, dry assemble, and check the joint closes fully and the faces are flush. Correct now, because after glue it cannot be corrected.
Glue the holes, not just the dowels.A dab in each hole and a smear on the dowel. Fluted dowels let excess glue and trapped air escape; a plain dowel in a blind hole can hydraulic-lock and split the wood.

Tips

Fluted, chamfered dowels are worth the small extra cost. The flutes carry glue and let air out, and the chamfer finds the hole.
If a dowel is a hammer-tight fit dry, it will split the part when the glue swells it. It should push in with firm thumb pressure.
Dowel centre points cost very little and turn dowelling from a precision exercise into a reliable one. They are the single best purchase for this joint.

Common mistakes

Measuring the hole positions on both parts separately — Two measurements can only agree by luck. Transfer instead — via a jig registered on both parts, or via centre points.
Drilling by eye without a guide — The eye is good at left and right and poor at forward and back. An out-of-square hole angles the dowel and the joint will not pull up.
Using dowels that have been left loose in a drawer — They absorb moisture, swell, and then will not enter a correctly sized hole — or they enter and split the wood as the glue swells them further.

If it doesn't work

The joint will not close by a millimetre or two

Cause: A hole not deep enough, or chips left in the bottom — Fix: Take it apart, clear the holes, and check the dowels are shorter than the combined hole depth. Shorten a dowel rather than deepening a hole near a face.

The parts are offset when the joint closes

Cause: Jig referenced off different faces, or the parts moved between marking and drilling — Fix: Nothing after glue-up but planing flush. Always reference the marked face on both parts.

The wood split when the dowel went in

Cause: Too tight, too large a dowel, or a blind hole with nowhere for the glue to go — Fix: Cramp and glue the split closed. Use fluted dowels and a slightly deeper hole next time.

Questions people ask

How many dowels does a joint need?

At least two on anything that could rotate, which is nearly everything. A single dowel is a pivot, not a joint. On a wide joint, space them evenly and keep them well in from the ends.

Are dowels stronger than biscuits?

Yes, meaningfully. A dowel puts long grain into long grain across the joint and resists shear and racking. A biscuit is mostly an alignment device. The trade is that dowels demand accuracy biscuits do not.