Mortise & Tenon Strength Calculator | Clifton Ellis's Workshop

A joint is where trust begins. In Roseville, I taught a man named Marcus to cut his first tenon in 1987. He's still using that joint today to support his grandson's treehouse. On Mars, the crew won't have Marcus — they'll have this calculator, and the numbers inside it.

This tool predicts the shear strength of a mortise-and-tenon joint based on wood species, grain orientation, moisture content, and joint geometry. No metaphors. No golden seams. Just the mathematics that keeps a seal from failing when the wind hits 150 miles per hour.

Joint Analysis

Species Modulus of Rupture —

Shear Strength (Parallel to Grain) —

Predicted Joint Capacity —

Safety Margin —

Failure Probability —

Interpretation:

Worked Example: White Oak Dome Frame

Scenario: A crew member cuts a mortise-and-tenon joint from white oak (MC 12%) to support a 400 kg habitat module section. Tenon depth: 30 mm. Shoulder width: 75 mm. Perfect grain alignment.

Calculation:

White oak MOR: 110 MPa (Forest Products Lab, 2010)
Shear strength parallel to grain: 9.8 MPa
Effective area: 30 × 75 = 2,250 mm²
Base capacity: 9.8 × 2,250 = 22,050 N ≈ 2,250 kg
Safety factor applied: 3.5×
Rated capacity: 643 kg

Result: Joint rated for 643 kg. Actual load: 400 kg. Safety margin: 60.75%. Status: GREEN LIGHT.

Data Sources & Citations

All mechanical properties sourced from:

U.S. Forest Products Laboratory. Wood Handbook: Wood as an Engineering Material, 2nd ed., General Technical Report FPL-GTR-190, Madison, WI: USDA, 2010.
Wikidata Q469555 (Quercus alba) — taxonomic verification
Wikidata Q76005 (Ultimate tensile strength) — mechanical property definition
NIST Materials Data Repository — moisture content correction factors v3.2

Agent-legible data: Full property constants published at joint-constants.json

Joint Parameters

Joint Analysis

Worked Example: White Oak Dome Frame