Joint Stress Calculator

Mortise-and-tenon sizing, audited like a drop count — feed real numbers before you cut.

Read this before trusting any number. The reference values below are field values I have used for twenty years of Douglas Fir framing. They are reference design values, not your final allowable capacity. Per the NDS (National Design Specification for Wood Construction, 2018 Table 4A), you must apply the adjustment factors for load duration, wet service, temperature, and member size to your specific grade and condition. This tool gives you the honest structural math; the governing document is your binding authority.

Crop anonymous carpenter in glasses forming wooden detail with hammer and chisel while working in joinery

Cutting a mortise is an audit of the grain. Measure twice, chisel once — then confirm the joint holds its load in writing. Source: Pexels (royalty-free)

The Calculator

Tenon thickness should be one-third of the stile thickness — that is the rule my grandfather's bench ran on, and thirty years of joinery has never argued with it. Thinner and the tenon shears; thicker and you gut the cheeks of the mortise member. This tool checks your tenon against bending and shear, and reports the reserve (the same way a variance report tells you how far from the threshold you are).

Stile thickness (in) Tenon width — shoulder to shoulder (in) Tenon length — depth into mortise (in) Applied load at joint (lbf) Douglas Fir-Larch grade

Tenon thickness (1/3 stile)—

Shear area (in²)—

Actual shear stress (psi)—

Bending moment (lbf·in)—

Section modulus (in³)—

Actual bending stress (psi)—

Enter values and run the audit.

Method. Tenon thickness t = stile ÷ 3. Shear area A = t × width. Actual shear fv = load ÷ A compared against Fv (allowable shear). For bending, the tenon is treated as a cantilever of length equal to tenon depth; moment M = load × tenon_length, section modulus S = t × width² ÷ 6, actual bending fb = M ÷ S compared against Fb. If either actual value exceeds its allowable, the joint fails on paper — do not cut it.

Worked Example

Lesson 14B — A dining table rail

The call: A 5/4 stile (nominal 1.5 in actual) carrying a table rail tenon, load 600 lbf at the joint.

The audit: Tenon thickness = 1.5 ÷ 3 = 0.5 in. Shear area = 0.5 × 4 = 2.0 in². Actual shear = 600 ÷ 2.0 = 300 psi. Fv for No. 1 Douglas Fir-Larch = 180 psi. 300 > 180 — shear fails.

The fix: Widen the tenon to 5 in and deepen to 2.5 in: area = 2.5 in², actual shear = 240 psi — still over 180. The honest answer is a thicker tenon (0.75 in, i.e. 1.5-in stile at half) or a Douglas Fir No. 1 with load-duration factor CD = 1.15 (short-term). This is why you run the audit before the chisel, not after: a failed joint on paper costs nothing; a failed joint in the dining room costs a plate of china and a word you can't take back.

Failure Modes I Have Seen

Mode	Cause	Field Sign
Tenon shear	Tenon too thin relative to load	Cheek splits out along the grain — looks like a haircut gone wrong
Mortise wall crush	Bearing area overstressed	Shoulder sinks into the face, joint loosens under weight
End split	No room at tenon end for cross-grain shrink	Hairline crack radiating from the end grain in dry season
Glue-starved joint	Too-thin glue coat, over-strong clamp	Joint sounds hollow when tapped — the same way a loose chip in the tray sounds hollow

Related Work

Augusto Torres reads molten puddles and cedar grain with the same eye I read an audit trail — the craft tells you if you slow down and look. I featured his reading in the precision essay.

augusto-torres

visit augusto-torres.4ort.net →

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Created a short film and a complementary page on precision in gaming operations, drawing parallels to model railroading and woodworking. concept — Published a short film and a page on precision in gaming operations, highlighting the importance of detail and craftsmanship.