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Vriksai Timber Intelligence

Mortise & Tenon Calculator

Calculate mortise and tenon dimensions using the universal 1/3 rule. Rail, stile, shoulder, cheek and mortise depth — covering IS 883, NDS, Eurocode 5, AS 1720.

Why this calculation matters

Joint proportions decide whether the rail or the mortise wall fails first. The one-third rule balances the two — thicker tenons are not stronger joints.

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Mortise & Tenon — Calculator

Calculate mortise and tenon dimensions using the universal 1/3 rule. Rail, stile, shoulder, cheek and mortise depth — covering IS 883, NDS, Eurocode 5, AS 1720.

Joint Type & Application
Member Dimensions
mm
mm
mm
mm

⚙ Mortise & Tenon Calculator

The world's oldest wood joint — found in ancient Egyptian furniture (3000 BC), traditional Indian carpentry, European timber framing and Japanese temple construction. The tenon (projecting tongue) fits the mortise (rectangular hole), creating a strong connection resisting racking, tension and bending.

Where Used?

Door & Window FramesChair & Table LegsPost & BeamCabinet FramesStaircaseTraditional Indian Furniture

Formulas (1/3 Rule — IS 883, NDS, Eurocode 5, AS 1720)

Tenon thickness = Rail thickness / 3Tenon width = Rail width − 2 × shoulder_allowanceTenon length = 4.5 × tenon_thickness (max = 80% of stile thickness)Mortise depth = Tenon length + 2mm clearanceCheek = (Rail thickness − Tenon thickness) / 2

About the Mortise & Tenon Calculator

The mortise and tenon is the backbone of solid timber joinery — a tenon (a projecting tongue) fits snugly into a mortise (a matching cavity) to make a joint that resists racking and pull-apart far better than nails or screws alone. The proportions matter: too thin a tenon is weak, too thick and the mortise walls crack. This calculator applies the time-tested 1/3 rule, sizing the tenon at a third of the rail thickness so strength is balanced on both sides of the joint, the same approach used in standards like IS 883, NDS 2018, Eurocode 5 and AS 1720. It applies the classic one-third tenon thickness rule and suggests sensible mortise and tenon proportions for your stock size.

Where Is This Used?

Furniture MakersDoor & Window FramesTimber Frame BuildersTraditional JoineryIndian Carpentry
Sources & verification
  • Universal one-third rule (IS 883, NDS 2018, Eurocode 5, AS 1720)

Formula checked against the sources above by an automated regression test (tests/verify-phase2.js) that derives each expected value independently of this page. Last reviewed .

FAQ

What is the 1/3 rule?
Tenon thickness = 1/3 of rail thickness. This leaves 1/3 for the mortise cavity and 1/3 cheek each side — balancing joint strength with structural integrity of the mortised piece. Used globally across IS 883, NDS 2018, Eurocode 5 and AS 1720.
Used in Indian carpentry?
Yes — called "chool aur khana" in some regions. Foundation of traditional Indian furniture in Rajasthan, Gujarat and South India. Modern craftsmen use identical 1/3 rule to international standards.
Should the tenon be glued along its whole length, or is a snug fit enough?
A good mortise and tenon gets most of its strength from a close fit plus glue on the cheeks — the broad faces of the tenon — because that's where the long-grain-to-long-grain glue bond is strongest. The shoulders matter too: they close the joint and resist racking, so they need to seat tightly. What you don't want is a tenon so tight it scrapes all the glue off going in, or so loose it rattles. Aim for a fit that slides home with hand pressure and holds a test piece without falling out — that's the sweet spot this calculator's proportions are built around.
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Wood species data

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How the calculation works

The one-third rule balances the two ways the joint can fail: a thinner tenon shears, a thicker one leaves too little mortise wall and the cheeks blow out instead. Everything is proportioned from the rail, not from the chisel you happen to own.

Show the formula
Tenon thickness = rail thickness / 3, rounded. Shoulder = 6, 10 or 14 mm by load class. Tenon width = rail width - 2 x shoulder. Tenon length = the lesser of 4.5 x tenon thickness and the available depth. Cheek waste = (rail thickness - tenon thickness) / 2. Mortise depth = tenon length + 2 mm.

Traditional joinery proportions, not a structural code. Species strength, glue type and whether the joint is pegged or wedged all change what it will actually carry.

References and what each one provides

  • EN 1995-1-1 (Eurocode 5)CENDesign of timber structures: fastener spacing and edge distances, connection capacity, and deflection limits.
  • IS 883Bureau of Indian StandardsIndian code of practice for design of structural timber, including permissible stresses, joint design and deflection limits.

Unverified values are marked as such rather than presented as sourced.

Limitations of this calculation

Sizes the joint by proportion rules. It does not account for species strength, glue type, load direction, shoulder fit quality, or whether the joint is pegged, wedged or draw-bored.

Do not use this for Structural frame design, or joints carrying live loads in buildings.

Figures are engineering estimates from the inputs and assumptions shown. Verify against the actual material and, where the result affects structure or safety, against a qualified professional.

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How to use this tool

  1. Enter your stock sizes below, then press Calculate.
  2. Pick your load category from the dropdown.
  3. Fill in rail width, rail thickness, stile thickness, rail span.
  4. Press Calculate.

Worked example

With the tool's starting values — rail width = 70, rail thickness = 45, stile thickness = 45, rail span = 600 — pressing Calculate gives: 50 mm mm (tenon width); 15 mm mm (tenon thickness (1/3)); 36 mm mm (tenon length).

Common mistakes to avoid