# Wood Beam Span Calculator

URL: https://speedycalculator.com/calculators/construction/wood-beam-span/
Last updated: 2026-10-25

> The wood beam span calculator estimates the maximum span of a sawn-lumber, LVL or glulam beam for a uniform load, limited by both bending stress and a deflection limit such as L/360, using NDS reference design values.

## Inputs

- Species and grade
- Beam size
- Uniform load on beam (lbf/ft (plf), kN/m, N/m)
- Deflection limit

## Results

- Section modulus
- Moment of inertia
- Span limited by bending
- Span limited by deflection
- Maximum span
- Maximum span (ft-in)

## Formula

```
Bending: L = √(8 × Fb' × S ÷ w), Fb' = Fb × size factor
Deflection: L = ∛(384 × E × I ÷ (5 × w × k)) for a limit of L/k
Maximum span = the smaller of the two
```

## Explanation

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

FieldWhat to enterSpecies and gradeChoose one: Douglas fir-larch No. 2, Southern pine No. 2, Spruce-pine-fir No. 2, Hem-fir No. 2 …Beam sizeChoose one: 2×6, 2×8, 2×10, 2×12 …Uniform load on beamUnits: lbf/ft (plf), kN/m, N/mDeflection limitChoose 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)15013.7222511.23009.73758.684507.926006.86

## Worked 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.

## FAQ

**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.


## Sources

- [American Wood Council – NDS Supplement design values](https://awc.org/)