How to use mortise and tenon dimensions
What this calculation does
A centered rectangular tenon leaves equal shoulders on each face and edge, while its length stops short of the blind mortise bottom.
Measure actual stock, tenon dimensions and mortise depth, then choose the bottom clearance for the joint you are making.
Enter the measurements
- Actual stock thickness: Measure actual stock thickness.
- Actual stock width: Measure actual stock width.
- Tenon thickness: Enter tenon thickness across the stock faces.
- Tenon width: Enter tenon width across the stock edge.
- Mortise depth: Measure blind mortise depth.
- Clearance at mortise bottom: Enter space left at the mortise bottom.
Formula and variables
Centered face shoulder = (stock thickness − tenon thickness) ÷ 2. Centered edge shoulder = (stock width − tenon width) ÷ 2. Tenon length = mortise depth − bottom clearance.
Each centered shoulder is half the difference between stock and tenon dimensions. Tenon length subtracts bottom clearance from mortise depth.
Read the result
The example leaves 0.25 in shoulders on both axes and an 0.875 in tenon length.
Worked example
Inputs: 0.75 × 2 in stock, 0.25 × 1.5 in tenon, 1 in mortise and 0.125 in bottom clearance
- Face shoulder = (0.75−0.25)÷2 = 0.25 in; edge shoulder = (2−1.5)÷2 = 0.25 in.
- Tenon length = 1−0.125 = 0.875 in.
Expected result: 0.25 in face and edge shoulders; 0.875 in tenon length
Frequently asked questions
Does this choose a strong tenon proportion?
No. It checks the entered geometry and does not determine joint strength.
Is the displayed clearance a glue-gap rule?
No. It is bottom clearance for the stated blind mortise only.
Conditions, presets and technical notes
Assumes a centered rectangular tenon with positive shoulders and a blind mortise. It does not determine joint strength, grain suitability, fit tolerance, glue gap, or a universal tenon proportion; use the actual cutter and test the fit.