🪵 Wood Beam Span Calculator

What This Calculator Does

This calculator estimates the longest span a simply-supported rectangular wood beam can safely carry under a uniform load, checking the two limits that typically govern beam design: bending strength and deflection stiffness.

The Formula

The bending-limited span comes from setting the maximum bending moment (wL^2/8 for a uniformly loaded simple beam) equal to the beam’s moment capacity (allowable bending stress Fb times section modulus S = bd^2/6), then solving for L. The deflection-limited span comes from the standard simple-beam deflection formula (5wL^4/384EI) set equal to a common L/360 deflection limit, then solved for L using the beam’s moment of inertia I = bd^3/12. The smaller of the two spans is the governing, maximum safe span.

Good to Know

• Allowable bending stress (Fb) and modulus of elasticity (E) vary by wood species, grade, and load duration — use published design values for your specific lumber (such as NDS reference design values) for real projects.
• This is a simplified planning estimate for general/educational use, not a substitute for a stamped structural engineering calculation, especially for concentrated loads, notched beams, or code-required load combinations.