How to use this calculator
- Enter stock dimensions. Use the actual stock thickness and the rail or member width across the tenon face.
- Choose tenon geometry. Enter finished tenon thickness, shoulder width each side and tenon length.
- Add fit allowances. Set cheek-side clearance, end clearance and bottom clearance for your tool setup and glue allowance.
- Cut and test fit. Use the calculated cheek cut depth, tenon width and mortise dimensions as a starting point, then verify on scrap from the same stock.
How it works
The calculator starts from the finished tenon. Tenon width is the member
width after subtracting both shoulders:
W_t = W - 2S
where W is the rail or member width and S is the
shoulder width on each side.
Cheek cuts center the tenon in the stock thickness:
d_c = (T - t_t) / 2
where T is stock thickness and t_t is finished
tenon thickness. The tenon thickness ratio is
t_t / T x 100
The mortise is then sized from the tenon plus allowance:
W_m = t_t + 2C_s L_m = W_t + 2C_e D_m = L_t + C_b
where C_s is cheek-side clearance, C_e is end
clearance and C_b is bottom clearance.
Pair the result with the cut list calculator for repeated rails and stiles, the dovetail spacing calculator for drawer joinery, the wood movement calculator for seasonal fit checks and the cabinet door size calculator when the joint belongs to a door or face-frame workflow.
Worked example
Verified against the live calculator
For 3/4 in stock, a 3 in rail, a
1/4 in finished tenon, 1/4 in shoulders and a
1-1/4 in tenon length, the tenon is
33.3% of stock thickness. The cheek cut depth is
0.250 in per side and the finished tenon width is
2.500 in. With 0.003 in side clearance,
0.015 in end clearance and 1/16 in bottom
clearance, the mortise screens at about 0.256 x 2.530 x 1.313 in.
Frequently asked questions
How thick should a tenon be?
A common first layout is roughly one third of the stock thickness, but final size depends on stock, tools, joint style and strength needs. This calculator reports the tenon thickness ratio and cheek cut depth so the setup is visible.
How do I size the mortise from the tenon?
Use the finished tenon thickness and width, then add small fit allowances. Mortise width is tenon thickness plus cheek-side clearance; mortise length is tenon width plus end clearance; mortise depth is tenon length plus bottom clearance.
Does this calculate mortise and tenon strength?
No. It is a dimensional layout screen. Joint strength depends on species, grain, glue, shoulder bearing, racking load, tenon length, fit, drawbore or wedge details and the actual furniture or frame design.
Should the mortise be deeper than the tenon?
Usually yes, by a small bottom clearance so the tenon can seat fully and glue or tool-bottom irregularity does not hold the shoulder open. The bottom clearance ratio output keeps that allowance visible.
Method & assumptions
- Tenon is assumed centered through the stock thickness, with equal cheek cuts on both faces.
- Shoulders are assumed equal on both sides of the tenon width.
- Mortise dimensions include entered fit clearances; they are not tool recommendations.
- Glue contact area is side-wall contact only and is not a strength rating.
- Does not model haunched tenons, wedges, drawbore pins, angled shoulders, loose tenons, domino-style tooling, racking strength, wood movement, grain runout, glue selection, tool deflection or production fixtures.