How to Use Wood Beam Span Calculator
The Wood Beam Span Calculator walks you through a simple-span check in minutes.
- Choose units and species — Imperial (in / psi) or metric (mm / MPa) with NDS-style defaults.
- Enter beam geometry — Width, depth and span length.
- Enter load — Total uniform load in plf or N/m, or convert from psf using tributary width.
- Adjust allowable stresses — Fb (bending), Fv (shear) and E (modulus) appear from species; tweak for grade or treatment.
- Read the verdict — Bending stress, shear, deflection and a pass / fail comparison.
Formula & Theory — Wood Beam Span Calculator
The Wood Beam Span Calculator uses classic mechanics for rectangular cross-sections:
S = b × d² / 6
I = b × d³ / 12
M_max = w × L² / 8
V_max = w × L / 2
fb = M_max / S
fv = 1.5 × V_max / (b × d)
δ = 5 × w × L⁴ / (384 × E × I)| Symbol | Meaning |
|---|---|
| b | Beam width |
| d | Beam depth |
| L | Span length |
| w | Uniform load per unit length |
| S | Section modulus |
| I | Moment of inertia |
| fb | Computed bending stress |
| fv | Computed shear stress |
| δ | Mid-span deflection |
A beam passes when fb ≤ Fb, fv ≤ Fv and δ ≤ L / DL (typically L/360 for live load).
Adjustment factors
For load duration (Cd), size (Cf) and wet service (Cm), modify Fb and Fv before entering them. The Wood Beam Span Calculator focuses on the geometry / load check.
Use Cases for Wood Beam Span Calculator
- Deck design — Verify joists and beams for residential decks.
- Framing — Check rafters, ridge beams and headers in light-frame construction.
- Renovations — Confirm whether an existing beam can carry an added load.
- Pergolas and outbuildings — Size lumber for sheds, pergolas and carports.
- Education — Teach the relationship between cross-section, span and deflection.
- Permits — Provide a transparent calculation to attach with permit drawings.
For final design, always defer to a licensed structural engineer. The Wood Beam Span Calculator is a fast and clear preliminary check.