Mortise and Tenon Size Calculator

The general rules around tenons are pretty straight forward. For example, most woodworkers follow this rule: the tenon should be roughly one third as thick as the rail it’s attached to. This is because glue surface area on the long grain provides strength. The appearance seems balanced but that factored in makes joint hold up over time. There is more to balance than.

See below, this calculator do all that work for you for whatever size and type of wood you have: No more guessing around with chisels. You don’t have to wonder what size your chisel should be. Now the calculator tell you exactly what area of glue you’re going to be covering. And why does that matter? Because glue doesn’t shear, wood fibers do. That’s how a joint fails.

How to Make Strong Wood Joints

It is better to make those cheeks as wide and long as possible. So imagine: You cinch down your drawbore peg. The angle on the hole yanks the tenon up into shoulder and compression holds it there, even if you didn’t get great glue. However: Cut too much off-set into a brittle wood (like hard maple) and stile will split, especially if you’re pulling too hard. By considering your wood type’s Janka hardness, the tool help you determine where the sweet spot lies.

Pine is softer, so it can withstand more drag before splitting; dense woods like oak and maple holds together well in shear force but are susceptible to shock failure under a lot of clamp pressure, so you don’t want too-aggressive an offset with them. These joints is sensitive to seasonal change, so beware. You’ll sometimes get surprised by how far apart those legs can move. If you cut your blind mortise too deep into the end grain of leg, the tenon will resist the wood’s seasonal shrinking and swelling which can cause it to crack. Because the tenon acts like a plug against this, the wood will crack.

Leave some back wall behind the mortise (enough to keep it strong but not so much that it blocks the breathing of face grain). This is something most calculators would of alert you to if your chosen length uses too much of depth of the stile. It saves you from cutting into nothingness…or creating a pocket that snaps under load.

This is where beauty meets utility. The shoulders are wide, which buries the edges of mortises well to match Shaker’s simple aesthetic. Yet shoulders are only as wide as they can be; any wider than your narrow rail allows and you’ve compromised the integrity of the tenon itself. There isn’t sufficient meat remaining between cut-outs to hold joint together.

So how do we find a happy medium? We find it by balancing not just the eye-line but also actual mass of material involved. We don’t want a joint so symmetrical it’s weak, but clean looking, either.

Testing the fit is important because nothing can substitute for feeling the wood slip easly into the carved-out pocket. With scrap cut from same lot, test fit by sliding into place. Too loose? Swing your mallet. Too tight? Chisel needs re-sharpening. Proper fit should slide easily into place with light clamp force or hand pressure. That resistance let you know the glue will hold and won’t tear fibers when assembled.

“Joinery is a conversation between method and material. Listen to the grain direction and density of the wood; let it tell you how to cut. The numbers help frame that dialogue, but your hands confirm it. At the end of the bench, there should be nothing about the joint you don’t like: It looks seamless enough to ignore and feels solid enough to trust with your weight. That balance gives furnitures lasting life beyond trends.”