A beam must be strong enough not to break (bending) and stiff enough not to sag (deflection). Short, heavily loaded beams are usually limited by bending; longer beams by deflection. This simplified check uses NDS reference values for common species and grades, the size factor for dimension lumber and a simply supported uniform load – it ignores load duration, wet service, lateral stability and shear, so final designs should use the code tables or an engineer.
How to use the Wood Beam Span Calculator
- Species and grade
- Beam size
- Uniform load on beam
- Deflection limit
Results update as you type, and every field accepts the units shown in its menu.
How it works
Choose the species/grade and the beam size, enter the uniform load along the beam (live + dead, per foot) and the deflection limit.
Tips for an accurate estimate
- Keep units consistent – the calculator handles conversions, but check every field’s unit before reading a result.
- Use values from standards, data sheets or test pieces whenever you have them; typical values are only a starting point.
- Apply appropriate safety factors and have structural, electrical and pressure work checked by a qualified professional.
- Record the inputs you used so the calculation can be reviewed later.
Common mistakes to avoid
- Confusing diameter and radius, or nominal and actual sizes.
- Using allowable stresses and ultimate strengths interchangeably.
- Treating rule-of-thumb results as a substitute for code tables or an engineered design.
Inputs explained
| Field | What to enter |
|---|---|
| Species and grade | Choose one: Douglas fir-larch No. 2, Southern pine No. 2, Spruce-pine-fir No. 2, Hem-fir No. 2 … |
| Beam size | Choose one: 2×6, 2×8, 2×10, 2×12 … |
| Uniform load on beam | Units: lbf/ft (plf), kN/m, N/m |
| Deflection limit | Choose one: L/360 (floors, live load), L/240 (roofs, total load), L/480 (stiff floors), L/720 (very stiff) … |
What each result means
- Section modulus = [b]×[d]^2÷6
- Moment of inertia = [b]×[d]^3÷12
- Span limited by deflection = (384×[E]×[Moment of inertia]÷(5×[Uniform load on beam]×[Deflection limit]))^(1÷3)
- Maximum span = min([Span limited by bending], [Span limited by deflection])
- Maximum span (ft-in) = [Maximum span]÷0.0254
Example values
How the main result changes when Uniform load on beam changes and everything else stays at the example values above:
| Uniform load on beam (lbf/ft (plf)) | Maximum span (ft) |
|---|---|
| 150 | 13.72 |
| 225 | 11.2 |
| 300 | 9.7 |
| 375 | 8.68 |
| 450 | 7.92 |
| 600 | 6.86 |
Formula
Bending: L = √(8 × Fb' × S ÷ w), Fb' = Fb × size factorDeflection: L = ∛(384 × E × I ÷ (5 × w × k)) for a limit of L/kMaximum span = the smaller of the twoWorked example
Inputs — Species and grade: Douglas fir-larch No. 2; Beam size: 2-ply 2×10; Uniform load on beam: 300 lbf/ft (plf); Deflection limit: L/360 (floors, live load).
Results — Section modulus = 42.8 in³; Moment of inertia = 197.9 in⁴; Span limited by bending = 9.7 ft; Span limited by deflection = 11.61 ft; Maximum span = 9.7 ft; Maximum span (ft-in) = 9 ft 8 3/8 in.
Key terms
- Fb
- Reference bending design value.
- E
- Modulus of elasticity.
- plf
- Pounds per linear foot of load along the beam.
Frequently asked questions
How far can a doubled 2×10 span?
Carrying 300 plf in Douglas fir No. 2, about 9–10 ft by this simplified check; IRC girder tables give similar values.
Why are there two spans?
One for strength (bending) and one for stiffness (deflection); the beam must satisfy both.
When should I use LVL?
LVL and glulam are stronger, stiffer and straighter than sawn lumber, so they span further and are common for headers and beams over 10 ft.
What formula does the Wood Beam Span Calculator use?
Bending: L = √(8 × Fb' × S ÷ w), Fb' = Fb × size factor.