Construction

Wood Beam Span Calculator

Estimate max simple-beam span from allowable stress.

Calculated Output

Max span 10

Max span 120

Formula: L = √(96·Fb·S ÷ w)

Step-by-Step Breakdown

1. Values used:

2. Allowable bending stress (Fb) = 1,200 psi

3. Section modulus (S) = 15 in³

4. Uniform load = 120 lb/ft

6. Max span = 10

7. Max span = 120

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The math behind it

Based on bending: max span L = √(96 × allowable stress × section modulus ÷ uniform load), from M = wL²/8 = Fb·S.

Fb 1200 psi, S 15 in³, 120 lb/ft → ~10 ft.

This isn't final sizing?

Only bending is checked here — shear, deflection and connection capacity also matter.