Beam Load Calculator
Calculate the maximum load capacity of a beam based on span, load type, and material.
Useful for structural planning and engineering estimates.
How Beam Load Calculations Work
Beam load calculations determine whether a structural beam can safely carry the loads placed on it without deflecting excessively or failing. There are two primary checks: bending stress and deflection.
Uniform load on a simply supported beam:
Maximum Bending Moment (M) = w × L² ÷ 8
Where:
- w = uniform load per linear foot (lb/ft)
- L = span length in feet
Maximum deflection formula:
δ = (5 × w × L⁴) ÷ (384 × E × I)
Where:
- E = modulus of elasticity (psi) — for Douglas Fir: ~1,700,000 psi
- I = moment of inertia of beam cross-section (in⁴)
- L must be in inches for this formula
Worked example — floor beam:
- Span: 12 ft (144 inches)
- Uniform load: 50 lb/ft (40 live + 10 dead)
- Beam: 2×10 lumber, E = 1,700,000 psi, I = 98.9 in⁴
δ = (5 × (50/12) × 144⁴) ÷ (384 × 1,700,000 × 98.9) δ ≈ 0.40 inches
Allowable deflection limit (L/360 for floors):
Max deflection = L ÷ 360 = 144 ÷ 360 = 0.40 inches
This beam is exactly at the code limit — borderline acceptable for a floor.
Common beam load types:
- Dead load: Permanent weight (framing, flooring, roofing) — typically 10–20 lb/ft²
- Live load: Variable weight (people, furniture, snow) — 40 lb/ft² for floors, 20–50 lb/ft² for roofs
- Point load: Concentrated force at specific location (column, post)
Always use span tables from the American Wood Council or consult a licensed structural engineer for load-bearing applications.