Mortise & Tenon Calculator

Size tenon thickness, width, and length proportionally to the workpiece, and get matching mortise dimensions.

Accepts fractions: 2-1/2, 3/4, 1.75
Most common: 3/4″ (S4S hardwood)
Leg or stile that receives the mortise

Tenon

Thickness1⁄4"≈ ⅓ rail thickness
Width2"≈ ⅚ rail width
Length1"⅔ mortised stock thickness

Shoulders

Side (face) shoulders1⁄4"Each face — equal both sides
Top/bottom shoulders1⁄4"Each edge — equal both ends

Mortise

Width (chisel/bit size)1⁄4"Match tenon thickness exactly
Depth1 ​1⁄16"Tenon length + 1⁄16″ clearance
Remaining wall1⁄2"Stock beyond mortise

Cut the tenon first, then fit the mortise to match. Choose the nearest available chisel or router-bit size for the mortise width and adjust the tenon cheeks accordingly.

Proportioning rules

Traditional proportions that produce a strong, well-fitted joint:

  • Tenon thickness — ⅓ the thickness of the tenon stock. A ¾″ rail gets a ¼″ tenon.
  • Tenon width — ⅚ the rail width, leaving equal shoulders top and bottom. For rails wider than 3–4″ consider twin or divided tenons to limit wood movement across the grain.
  • Tenon length — ⅔ the thickness of the mortised member, leaving at least ⅜″ of wall beyond the bottom of the mortise to prevent splitting.

Always cut the tenon first and fit the mortise to it — it is far easier to pare a tight mortise to fit than to add material back to an undersized tenon. Choose the nearest chisel or router-bit size for the mortise width and adjust the tenon cheeks to match.