Mortise and Tenon Calculator
Joint dimensions, peg sizing, and shear capacity from timber size and load using traditional timber framing rules.
About this calculator
This calculator sizes a traditional mortise-and-tenon joint from the two timbers meeting at it, then checks whether the tenon's shear capacity covers the applied load. Tenon Thickness follows the classic 1/3-of-timber-width rule (rounded to the nearest half-inch, floored at 1.5 in), driven entirely by Tenon Timber Width. Tenon Width is the Tenon Timber Depth minus a fixed 1-inch shoulder on each side -- and because the calculator's minimum allowed timber depth (4 in) is exactly the point where that shoulder rule bottoms out, the resulting Shoulder Depth works out to a constant 1.0 in across the entire declared depth range, not something that grows with a larger timber. Tenon Length comes from 3/4 of the Mortise Timber Width, and Mortise Depth is that length plus a 1/4-inch clearance.
Peg Diameter is roughly a third of tenon thickness (minimum 3/4 in), and Minimum Relish -- the wood left beyond the peg hole to resist splitting -- is twice the peg diameter. Applied Joint Load never affects any of those geometry numbers: it only enters the picture through Shear Utilization Ratio, which divides the load by Tenon Shear Capacity (itself set by species shear strength and the tenon's own cheek area). The species shear values behind that capacity -- 180, 175, and 135 psi for Douglas Fir-Larch, Southern Pine, and Eastern White Pine respectively -- come directly from the American Wood Council's NDS Supplement. Draw-Bore Offset is reported as a fixed 1/16 in regardless of any input, reflecting the traditional rule rather than a load-dependent calculation.
Inputs
Results
Tenon thickness
2.5 in
Tenon width
6 in
≈ 2 credit cards
Figures current as of 2018. Source: American Wood Council, National Design Specification (NDS) Supplement: Design Values for Wood Construction, 2018 Edition, Table 4A/4B (Reference Design Values for Visually Graded Dimension Lumber)
How to Use This Calculator
- Enter Tenon timber width, Tenon timber depth, and Mortise timber width.
- Set Applied joint load and Wood species.
- Review Tenon thickness (in) and Tenon width (in).
- Use Tenon length (in) and Mortise depth (in) to inform your decision.
How the result changes with Tenon timber width
| Tenon timber width | Tenon thickness | Tenon width |
|---|---|---|
| 4 | 1.5 in | 6 in |
| 6 | 2 in | 6 in |
| 12 | 4 in | 6 in |
| 16 | 5.5 in | 6 in |
What each input means
- Tenon timber width
- Width of the timber that will have the tenon cut into it.
- Tenon timber depth
- Depth (height) of the timber that will have the tenon.
- Mortise timber width
- Width of the timber receiving the mortise (tenon inserts into this).
- Applied joint load
- Force the joint must resist (gravity load or lateral load).
- Wood species
- 0 = Douglas Fir-Larch, 1 = Southern Pine, 2 = Eastern White Pine.
What each result means
- Tenon thickness
- Recommended tenon thickness (≈ 1/3 timber width).
- Tenon width
- Tenon width (timber depth minus 1 in shoulder each side).
- Tenon length
- How far the tenon extends into the mortise (≈ 3/4 receiving width).
- Mortise depth
- Mortise depth (tenon length + 1/4 in clearance).
- Mortise wall thickness
- Wood remaining on each side of the mortise in the receiving timber.
- Shoulder depth
- Shoulder dimension on each side of the tenon.
- Peg diameter
- Recommended hardwood peg diameter (≈ 1/3 tenon thickness, min 3/4 in).
- Minimum relish
- Minimum wood beyond the peg hole to prevent splitting (2× peg diameter).
- Draw-bore offset
- Typical offset between tenon and mortise peg holes for draw-boring.
- Tenon shear capacity
- Maximum shear load the tenon cheeks can resist.
- Shear utilization ratio
- Applied load / shear capacity. Must be ≤ 1.0.
How this is calculated
Worked example, using the default values
- Identify Input Parameters4 parametersTenon timber width = 8, Tenon timber depth = 8, Mortise timber width = 8, Applied joint load = 2000 = 5 input(s) provided
- Calculate Tenon thicknessTenon thickness = max(1.5, round(rawTenonThickness * 2) / 2)2.5 = 2.5
- Calculate Tenon widthTenon width = max(2, timberDepthIn - 2)6 = 6
- Calculate Tenon lengthTenon length = max(2, round(receivingWidthIn * 0.75 * 2) / 2)6 = 6
- Calculate Mortise depthMortise depth = tenonLength + 0.256.25 = 6.25
Figures and sources
- Shear parallel to grain (Fv) reference design values for Douglas Fir-Larch, Southern Pine, and Eastern White Pine (2018) — American Wood Council, National Design Specification (NDS) Supplement: Design Values for Wood Construction, 2018 Edition, Table 4A/4B (Reference Design Values for Visually Graded Dimension Lumber)
Engine last updated . Checked against 2 independently-derived tests — how we verify calculators. Built by Paul Gunder, a software engineer, not a licensed financial, medical, or legal professional.
Frequently Asked Questions
Why does the Shoulder Depth stay at 1 inch no matter what I enter for timber depth?
Tenon Width is calculated as the Tenon Timber Depth minus a fixed 1-inch shoulder on each side, and the calculator's minimum allowed timber depth (4 in) sits exactly at the point where that rule's own floor would otherwise kick in. Across the full 4-24 in depth range the shoulder works out to exactly (depth - (depth - 2)) / 2 = 1.0 in every time -- it is a fixed 1-inch shoulder rule, not a proportional one, so it does not scale up on larger timbers.
Does a heavier Applied Joint Load change the recommended tenon dimensions?
No. Tenon Thickness, Tenon Width, Tenon Length, Mortise Depth, and Peg Diameter are all sized from the timber dimensions alone, using traditional proportional rules of thumb. Applied Joint Load only feeds into Shear Utilization Ratio (load divided by Tenon Shear Capacity) -- a heavier load raises the utilization ratio toward or past 1.0 without changing any of the joint's recommended dimensions, which is a signal you may need a larger timber or an additional joint, not just a bigger peg.
What determines the Tenon Shear Capacity?
Shear capacity comes from the wood species' shear strength parallel to grain (Fv -- 180 psi for Douglas Fir-Larch, 175 psi for Southern Pine, 135 psi for Eastern White Pine) multiplied by the shear area of both tenon cheeks (tenon width x tenon length x 2). Those three Fv figures are the American Wood Council's own published reference design values from the NDS Supplement, not rounded estimates. Shear capacity depends only on the timber geometry and species, never on the Applied Joint Load itself, which is why the same joint can show very different utilization ratios under different loads without its capacity number moving.
How is the recommended Peg Diameter chosen?
Peg Diameter is roughly one-third of the calculated Tenon Thickness, rounded to the nearest 1/8 inch, with a 3/4-inch practical minimum for traditional hardwood pegs. Since Tenon Thickness itself scales with Tenon Timber Width (thicker timbers need thicker tenons), a wider tenon timber generally calls for a larger peg -- Minimum Relish, the wood required beyond the peg hole to prevent splitting, is then set at twice that peg diameter.
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