Wood Beam Span Calculator

Estimate the safe span of a simply-supported wooden beam under a uniform load. Verify with span tables / an engineer.

How the Math Works

The Wood Beam Span Calculator uses the formula L = √(96·Fb·S ÷ w) to determine the maximum allowable span for a simple beam. This equation combines four critical engineering parameters: Fb (allowable bending stress), S (section modulus), w (uniform load per unit length), and a constant (96) derived from beam deflection theory. The square root operation ensures the result yields a linear span measurement, while the multiplicative relationship between Fb and S reflects how stronger materials and more robust cross-sections enable longer spans.

Practical Applications

To use this calculator, first determine the maximum allowable bending stress (Fb) for your wood species and grade from engineering tables, then calculate the section modulus (S) based on your beam's dimensions. Next, compute the total uniform load (w) in pounds per linear foot, accounting for both the beam's self-weight and any superimposed loads. Input these values into the formula to find the maximum span (L), which should always be rounded down to the nearest practical measurement to ensure safety margins are maintained.

Day-to-Day Use

This calculator helps homeowners, contractors, and DIY enthusiasts make informed decisions about wood beam sizing for decks, porches, sheds, and floor joists. By quickly estimating maximum spans, you can select appropriate beam sizes before purchasing lumber, avoid costly mistakes that require structural modifications, and ensure your projects meet building code requirements. Whether you're planning a weekend deck build or a major renovation, this tool saves time and materials while promoting safety and structural integrity.

Worked example

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

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