Wood Beam Design Calculator

The calculator doesn't carry a species-and-grade lookup table internally — it works off a single fixed 10 MPa reference bending value and expects you to supply the species factor as a multiplier relative to that baseline, per Table 4A of the AWC's 2018 National Design Specification (NDS) for Wood Construction. That's why the help text gives you ranges like Douglas Fir-Larch at roughly 1.0 and Southern Pine higher: you're converting the real NDS Fb value into a ratio against the generic 10 MPa the engine actually uses.

Depth, by a wide margin. Section modulus is S = bd²/6, so depth is squared while width is linear — doubling depth quarters the bending stress fb = M/S, while doubling width only halves it. That's the mathematical reason deeper joists and beams are so much more efficient at resisting bending than wider ones of the same cross-sectional area.

No — this is a bending-stress screen only. It compares actual fb against an adjusted allowable Fb and says nothing about shear capacity at the supports, deflection under service loads, or connection design, any of which can govern before bending does, especially on shorter or heavily loaded spans. It also assumes a single uniform load with no lateral-torsional buckling reduction, so treat a pass here as one necessary check among several, not a finished design.

CM represents wet-service conditions — moisture content above 19% for sawn lumber — which weaken wood's bending, stiffness, and shear properties relative to the dry-condition baseline the reference values assume. That's why CM is capped at 1.0 for dry use and drops below 1.0 (e.g. 0.85 for Fb) as service conditions get wetter: it's always a penalty on the adjusted allowable stress, never a bonus.