Wood Beam Span Calculator — Calculate Safe Span & Beam Size

This Wood Beam Span Calculator helps you determine the maximum safe span, deflection, and recommended beam sizes for various wood species and grades.

Distance between supports
lb/ft

Results

Disclaimer: This calculator provides simplified, reference-level calculations only. It does not replace design by a licensed structural engineer. Always verify final designs with a qualified professional.
Span Deflection exaggerated
PASS - Beam meets requirements
Maximum Recommended Span
16.2 ft
Actual Deflection
0.42 in (L/411)
Allowable Deflection
0.53 in (L/360)
Bending Stress
875 psi
Allowable Bending Stress
1200 psi
Safety Factor
1.37

Recommended Beam Sizes

How to Use the Wood Beam Span Calculator

Follow these steps to calculate the appropriate wood beam span for your project:

  • Select your preferred unit system (Imperial or Metric)
  • Choose whether to calculate maximum span or check a given span
  • Select the material type, wood species, and grade
  • Enter the beam dimensions (width and depth)
  • Input the span distance (if checking a specific span)
  • Select the load type and enter the load value
  • Choose the appropriate support condition and deflection limit
  • Select the load duration factor
  • Click "Calculate" to see results

How the Calculation Works

The Wood Beam Span Calculator uses standard engineering formulas to determine beam performance:

  • Max bending moment for UDL: Mmax = w × L² / 8
  • Max deflection for UDL: δ = 5 × w × L⁴ / (384 × E × I)
  • Section modulus: S = b × d² / 6
  • Bending stress: fb = Mmax / S
  • Shear stress: fv = 3 × V / (2 × b × d)

Where: w = distributed load, L = span length, E = modulus of elasticity, I = moment of inertia, b = beam width, d = beam depth, V = shear force.

Common Allowable Deflection Limits

Deflection Limit Typical Application
L/240 Roofs without plaster ceiling
L/360 Floors (typical residential)
L/480 Floors supporting plaster ceiling
L/600 Special applications requiring minimal deflection

Typical Wood Species Properties

Species Modulus of Elasticity (E) psi Allowable Bending Stress (Fb) psi
Douglas Fir 1,600,000 1,200
Southern Pine 1,600,000 1,250
Oak 1,300,000 1,000
Spruce 1,200,000 875
Hem-Fir 1,300,000 1,050

Examples

Example 1: Floor Joist Calculation

Inputs: Douglas Fir No.2 2×10 beam, UDL of 40 lb/ft, simply supported, L/360 deflection limit, normal load duration.

Results: Maximum span = 16.2 ft, actual deflection = 0.42 in, bending stress = 875 psi (73% of allowable).

Example 2: Roof Beam Check

Inputs: Southern Pine No.1 4×12 beam, 20 ft span, UDL of 50 lb/ft, simply supported, L/240 deflection limit.

Results: PASS - Beam meets all requirements with safety factor of 1.45.

FAQ

What is the difference between UDL and point load?

UDL (Uniform Distributed Load) is spread evenly along the beam, while a point load is concentrated at a specific location (usually the center for this calculator).

How does wood grade affect beam capacity?

Higher grades (Select Structural, No.1) have fewer defects and higher allowable stresses, resulting in greater load capacity and longer spans.

What deflection limit should I use?

Use L/360 for floors, L/240 for roofs without plaster ceilings, and L/480 for floors with plaster ceilings.

Can I use this calculator for deck beams?

Yes, but ensure you use appropriate load values for deck usage and consider environmental factors like wet service conditions.

Why is my beam failing in shear?

Shear failure typically occurs with short, deep beams under heavy loads. Try increasing beam width or selecting a stronger wood species.

How accurate is this calculator?

This tool provides estimates based on standard engineering formulas. For final designs, always consult a licensed structural engineer.