Beam Size Calculator
A beam size calculator estimates the minimum required dimensions for a structural beam based on the applied load, span, and material properties. Beams are horizontal structural members that carry loads perpendicular to their length, typically floors, roofs, or ceilings. Selecting an undersized beam can lead to excessive deflection or structural failure, while an oversized beam wastes material and increases cost. This calculator uses the fundamental bending stress formula to find the required section modulus for a simple simply-supported beam under a uniform load, then calculates the minimum depth and width needed. Enter the beam span, the total distributed load, and the material type to get a preliminary estimate. Always have a licensed structural engineer review any beam design before construction.
Beam bending formula
M = (w x L^2) / 8 [maximum moment for simply-supported uniform load]
S_required = M / Fb [required section modulus]
d = sqrt( (6 x S) / b ) [required depth for rectangular section]
Where: w = uniform load (lbs/in), L = span (in), Fb = allowable bending stress (psi), b = beam width (in), d = beam depth (in).
Beam design considerations
- Bending stress controls for longer spans with heavy loads
- Deflection (L/360 for live load) often controls beam depth for wood beams
- Shear stress should be checked at supports for short, heavily-loaded beams
- Lateral bracing is required to prevent lateral-torsional buckling in deep beams
- Connection design at supports is a separate critical calculation
Frequently asked questions
What does a beam size calculator do?
A beam size calculator determines the minimum required depth and width of a beam to safely carry a given load over a specified span without exceeding allowable bending stress limits for the selected material.
What is the section modulus of a beam?
The section modulus (S) relates a beam's bending moment to the bending stress. For a rectangular beam: S = (b x d^2) / 6, where b is width and d is depth. A larger section modulus allows the beam to resist greater bending moments.
What is allowable bending stress for wood beams?
Allowable bending stress for structural lumber varies by species and grade. Douglas Fir-Larch No. 1 has a typical allowable bending stress of about 1,500 psi. Always consult the National Design Specification for Wood Construction for exact values.
How does span length affect beam size?
Beam size requirements increase significantly with span. The required section modulus is proportional to the square of the span (for uniform loads), so doubling the span roughly quadruples the required beam size.
Should I use this calculator for actual structural design?
No. This calculator provides preliminary size estimates only. Actual structural beam design must be performed by a licensed structural engineer and conform to the applicable building code, including deflection limits, lateral stability, and connection design.
Official sources
- American Wood Council: National Design Specification for Wood Construction (NDS).
- American Institute of Steel Construction: AISC Steel Construction Manual.
Reviewed by the CalculatorHub team, edited by James Graham, 14 June 2026. See our methodology.