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.
How we build and check this calculator
This calculator runs entirely in your browser, so the numbers you enter stay on your device. The math behind it is written by hand and tested against worked examples and standard references before the page goes live.
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