What Dowel Sizes Mean for Joint Strength

Pick up a 6mm dowel and an 8mm dowel. The diameter difference feels minimal in your hand. But that 2mm changes the mathematics of the joint significantly. Not just because the dowel is thicker, but because of how that thickness interacts with hole depth, glue surface area, and the wood surrounding it.

Most dowel jigs come with bushings for 6mm, 8mm, and 10mm dowels. Those aren't arbitrary sizes. They represent different applications and different structural expectations.

Surface Area Increases Faster Than Diameter

A 6mm dowel at 30mm deep has roughly 565 square millimeters of glue surface area. An 8mm dowel at the same depth has about 754 square millimeters. That's 33% more gluing surface for a 33% increase in diameter. Double the diameter and you double the surface area, assuming depth stays constant.

But depth rarely stays constant. Thicker dowels typically go deeper because you're working with thicker stock. A 10mm dowel might extend 40mm into each piece. Now you're at 1,256 square millimeters of glue surface per side. More than double the 6mm dowel.

Glue creates the bond in dowel joints. Modern wood glues are stronger than the wood fibers themselves when properly applied. More glue surface means more total bonding strength, but only if the glue can actually reach all that surface. A gap of even a few thousandths of an inch between dowel and hole wall reduces effective glue contact.

The Mechanical Resistance Question

A dowel also provides mechanical resistance to shear forces. When a joint tries to slide apart parallel to the dowel axis, the dowel itself has to shear or the wood around it has to fail. Larger diameter means more material has to fail.

The cross-sectional area increases with the square of the radius. A 6mm dowel has a cross-sectional area of about 28 square millimeters. An 8mm dowel has 50 square millimeters. A 10mm dowel has 78 square millimeters. Nearly three times the cross-section of the 6mm dowel.

Birch dowels, the most common hardwood dowel material, have a shear strength around 6-8 MPa depending on grain orientation and moisture content. That 10mm dowel can theoretically resist about 470-625 newtons of shear force before the wood itself fails. The 6mm dowel maxes out around 170-225 newtons.

But joints rarely fail by shearing the dowel. They fail when the surrounding wood crushes or splits, or when the glue bond breaks. The dowel's mechanical strength is almost always greater than the weakest link in the system.

Stock Thickness Dictates Maximum Dowel Size

You can't put a 10mm dowel in 1/2" (12.7mm) stock. There's not enough material around the dowel to resist splitting. The wood surrounding the dowel needs sufficient thickness to contain the expansion forces when glue is applied and to resist crushing under load.

A general guideline exists: dowel diameter should be roughly one-third to one-half the stock thickness. In 3/4" (19mm) material, that suggests 6mm to 10mm dowels. In 1-1/2" (38mm) material, you could go to 12mm or even larger.

These aren't absolute rules. They're observations about what typically works before the surrounding wood fails. Hardwoods can support larger dowels relative to thickness because they resist crushing better than softwoods. Different wood species have different strengths in compression perpendicular to grain, which is exactly the stress dowel joints create.

Depth Matters More Than You'd Think

A dowel that's 25mm deep has less strength than the same diameter dowel at 40mm deep. The longer dowel has more glue surface and more mechanical engagement with the wood. But there's a point of diminishing returns.

Once the dowel extends deep enough that the glue bond strength exceeds the strength of the surrounding wood, going deeper doesn't increase joint strength. It just wastes dowel and increases the risk of drilling through the other side of your workpiece.

For 6mm dowels in typical hardwood, 25-30mm depth per side is usually sufficient. For 8mm dowels, 30-35mm works. For 10mm dowels, 35-40mm provides good engagement without being excessive. These depths assume the dowel is properly sized for the stock thickness.

Drilling too deep creates another problem: the hole bottom can trap glue and air, preventing the dowel from seating fully. A dowel that bottoms out before the joint closes leaves a gap on one side of the joint. That gap is visible and weakens the connection.

Multiple Dowels Change the Calculation

Two 6mm dowels don't equal one 12mm dowel. They equal two independent glue joints, each with its own surface area and mechanical resistance. The total strength is roughly additive, assuming proper alignment and spacing.

Face frame construction typically uses two 6mm or 8mm dowels per joint. Cabinet carcasses might use three or four 8mm dowels on wider pieces. The spacing between dowels matters. Too close together and you risk splitting the wood between them. Too far apart and the joint can twist or rack under load.

Dowel spacing also affects how loads distribute through the joint. Widely spaced dowels resist twisting better than closely spaced ones. But widely spaced dowels in narrow stock leave less wood at the edges, increasing the risk of edge splitting.

Glue Gap Tolerance

Dowels are typically sized to create a slight interference fit: the dowel diameter is a few thousandths of an inch larger than the hole. This ensures good contact for gluing. But manufacturing tolerances vary.

A 6mm dowel might actually measure 5.95mm to 6.05mm depending on the manufacturer and how much moisture the wood has absorbed. A 6mm drill bit might produce a hole anywhere from 6.0mm to 6.15mm depending on bit quality, sharpness, and drilling technique. That's potential for either too tight (splitting) or too loose (weak glue joint).

Smaller diameter dowels are more forgiving of these tolerance variations. A 0.1mm gap in a 6mm joint is a larger percentage of the circumference than the same gap in a 10mm joint. But in absolute terms, a 0.1mm gap reduces glue contact the same amount regardless of dowel size.

The flutes or grooves spiraled into most commercial dowels help with this. They provide glue channels and allow air to escape during assembly. But they also reduce the total glue surface area slightly. A smooth dowel would theoretically provide more glue surface, but it would be nearly impossible to assemble without trapping air.

The Plywood Edge Exception

Plywood changes everything. The alternating grain layers and multiple glue lines make edge doweling problematic regardless of dowel size. Smaller dowels create less expansion pressure, which reduces splitting risk, but they also provide less strength.

In plywood edges, dowel size becomes a compromise between having enough strength and not causing delamination. Many woodworkers find that 6mm dowels work better than 8mm or 10mm in plywood, not because of any strength advantage, but because they're less likely to split the face veneers.

Standard Sizes Exist for a Reason

The 6mm, 8mm, and 10mm progression isn't arbitrary. These sizes work with common stock thicknesses: 1/2", 3/4", and 1-1/4" material respectively. They're also metric equivalents of older imperial sizes that have been used for decades.

Larger dowels exist. 12mm, 15mm, even 20mm dowels are available for timber framing and heavy construction. Smaller dowels exist too: 3mm, 4mm, and 5mm dowels see use in musical instrument making and fine woodworking. But for general woodworking and furniture making, the 6mm-8mm-10mm range covers most applications.

Drill bit sizes align with these standard dowel diameters. Buying a dowel jig means buying specific bit sizes for those jig bushings. Once you commit to a system, you're working within that size range unless you want to start drilling freehand or buying specialized equipment.

What Actually Fails First

In a properly made dowel joint with correctly sized dowels, the glue bond rarely fails. The wood surrounding the dowel fails. Either it crushes under compression, splits under expansion, or tears when the joint is pulled apart.

This is why dowel size relative to stock thickness matters more than absolute dowel size. A 10mm dowel in 3/4" stock will split the wood. The same dowel in 2" stock works fine. The dowel didn't change. The amount of wood surrounding it changed.

Joint failure usually happens at stress concentrations: near edges, at knots, or where grain direction changes. Larger dowels create larger stress concentrations simply because they displace more wood. They also create more clamping pressure during glue-up as they expand from moisture.

Matching Dowel to Application

Face frames can use 6mm dowels because the joints don't see much stress. They're alignment joints as much as structural joints. Two small dowels per joint is sufficient.

Table legs and stretchers need 8mm or 10mm dowels because these joints experience real structural loads. The joint needs to resist racking forces and support weight. Larger dowels provide that strength.

Edge-to-edge joints for panels work with any size dowel that fits the stock thickness. Here the dowels are primarily for alignment during glue-up. The long-grain-to-long-grain glue joint provides most of the strength. The dowels just keep things from sliding during clamping.

Drawer boxes often use 6mm dowels because drawer construction is typically 1/2" material. The joints see stress from opening and closing, but not heavy structural loads. Small dowels are sufficient and reduce splitting risk in the thin stock.

The size that makes sense depends on what forces the joint will experience, how thick your stock is, what wood species you're working with, and whether you're joining solid wood or sheet goods. Those variables matter more than any rule about "always use 8mm" or "bigger is stronger."