Mortise and Tenon Proportion Calculator
The mortise and tenon joint is one of the most enduring joints in woodworking, used in everything from Windsor chairs to timber-frame structures. Getting the proportions right matters: a tenon that is too thick risks splitting the mortised piece, while one that is too thin is weak. This calculator applies the traditional one-third rule, sizing the tenon thickness to one-third of the rail stock thickness, with shoulders on each face. Enter the tenon stock thickness and width (the rail dimensions), and the mortised-piece thickness (the stile or leg), and the calculator returns tenon thickness, shoulder width, tenon length recommendation, and mortise dimensions.
Mortise and tenon proportion formula
Tenon thickness = rail thickness / 3
Face shoulder = (rail thickness - tenon thickness) / 2
Tenon width = rail width - 2 × (rail width × 0.1 + 0.125)
Tenon length = min(stile thickness × 0.8, 2.5)
The one-third rule sets tenon thickness at one-third of rail thickness, leaving equal shoulders on each face. The tenon width is narrowed by approximately 10 percent of the rail width plus a minimum 1/8-inch edge clearance on each side to prevent splitting. Recommended tenon length is 80 percent of the stile thickness, capped at 2.5 inches for typical furniture.
Mortise cutting tips
- Mark the mortise with a mortise gauge set to the tenon thickness for perfectly matched marks.
- Chop mortises with a bench chisel or drill out the waste with a Forstner or brad-point bit, then pare to the lines.
- Cut mortises before tenons so any slight variation in the mortise width is matched by trimming the tenon.
- Glue surface area is the primary strength driver: longer tenons and wider faces provide more bond area.
- For drawbore pegging, drill the tenon hole 1/16 inch closer to the shoulder than the mortise hole to create a mechanical clamp when the peg is driven.
Mortise and tenon calculator: frequently asked questions
What is the one-third rule for mortise and tenon joints?
The traditional rule of thumb is that the tenon thickness should be approximately one-third the thickness of the stock being joined. This leaves enough wood on each side of the mortise to avoid splitting, while making the tenon strong enough to resist the bending forces in service.
How long should a tenon be?
A common recommendation is that tenon length should be at least three times the tenon thickness, or roughly equal to the width of the mortised member. For frame-and-panel furniture a tenon length of 1 to 1.5 inches is typical. Longer tenons provide more glue surface and greater resistance to racking.
What is the difference between a through-tenon and a stopped tenon?
A through-tenon passes completely through the mortised piece and is visible on the far face. A stopped (blind) tenon ends inside the mortised piece. Through-tenons can be wedged for extra strength. Stopped tenons are neater and used where the joint should not show.
How much glue gap should I leave in a mortise and tenon?
For a well-fitted glued joint, the gap on each side of the tenon should be 0.003 to 0.005 inches (about the thickness of a sheet of copy paper). A joint that is too tight will shear the glue; one that is too loose reduces glue bond area.
Should the tenon width equal the width of the rail?
No. Leave at least 1/4 inch of shoulder on each side of the tenon width so the tenon does not extend to the very edge of the rail. This prevents tearout at the shoulder and provides glue area on the faces. For wide rails, tenon width can be 1/2 to 3/4 inch less than the full rail width.
Official sources
- USDA Forest Products Laboratory: Wood Handbook, FPL-GTR-190 (fastening and joining).
- USDA Forest Service: FPL Homepage.
Reviewed by the CalculatorHub team, edited by James Graham, 15 June 2026. See our methodology.