Note: What Dowel Size To Use for Wood Joinery

One of the first questions you may have when you begin using dowel joinery is: What dowel size should I use?

When woodworkers talk about dowel size most often they are referring to the diameter of the dowel. There is a general rule for diameter size (loosely based on mortise and tennon joinery) that will satisfy the most typical dowel joinery conditions. That is the rule of thirds and half.

The rule of thirds and half for wood dowel sizing states:

The diameter of the dowel should be no smaller than one-third (1/3) the actual thickness of the smallest board being joined. This ensures that the dowel is substantial enough to provide good holding power.
The diameter of the dowel should be no larger than one-half (1/2) the actual thickness of the smallest board being joined. This prevents the dowel from being too bulky or weakening the wood by removing too much material.
Ideally, the optimal dowel size falls halfway between the smallest and largest calculated sizes using the rule of on-third and rule of one-half. This balanced approach aims to provide a secure and aesthetically pleasing joint without compromising strength or wood integrity.

This rule serves to ensure there’s a sufficient amount of material between the edges of the boards and the dowel to minimize the risk of joint failure in the connected boards. The greater the material thickness between the dowel and the board’s sides, the lower the likelihood of encountering issues in the joined boards. Conversely, when using a smaller dowel, joint failure is more likely to occur within the dowel or the adhesive bond itself.

When opting for a larger dowel, although the dowel itself is stronger, there’s less original material remaining between the dowel and the board’s side. Consequently, the risk of joint failure with a larger dowel is more concentrated in the thinner sections of the board’s edges. Ideally, the best choice lies in a dowel size that falls midway between the largest and smallest recommended sizes, striking a balance between strength and material thickness.

Example of the rule of thirds or half:

Let’s say you want to join a 3/4″ thick board to another 3/4″ thick board (actual thicknesses):

The rule of thirds states that a dowel should be no smaller than one third the thickness of the smallest board. So 1/3 of 3/4″ would equal 1/4″.

The rule of half states that a dowel should be no larger than one half the thickness of the smallest board. So 1/2 of 3/4″ would equal 3/8″.

The optimal dowel size would be halfway between 1/4″ and 3/8″. Which would be 5/16″. Keep in mind that most dowel jigs (such as the Jessem 8550 doweling jig) do not offer a 5/16″ guide. So you must choose between the two. However, you must also consider the width of the board because of the rules of dowel spacing.