Wood Beam Span Calculator

How to use this wood beam span calculator

What this calculator does

This calculator estimates midspan deflection, maximum bending stress, and average shear stress for a simply supported solid-sawn rectangular wood beam carrying a uniformly distributed line load. It compares those demands with simplified reference design values for the selected species and grade. It is useful for early sizing, checking whether a proposed beam is obviously overstressed, comparing beam depths, and understanding which limit state governs. It is not a stamped structural design and does not verify connections, bearing, lateral stability, repetitive-member effects, notches, holes, fire resistance, vibration, snow drift, seismic effects, or every adjustment required by a building code.

When to use it

Use it while comparing a 2×10 with a 2×12 for a short opening, testing how a stricter deflection limit changes serviceability, checking how a higher line load affects utilization, or preparing questions for a structural engineer. For permitted construction, verify the result against locally adopted codes and current span tables.

How to calculate

1. The calculator opens with a complete demonstration: Select Structural Douglas Fir-Larch, a nominal 2×10 beam, an 8 ft span, a 240 lb/ft uniform load, and an L/240 limit. The example workbook is immediately available.
2. Choose Wood species and Lumber grade. These selections set the reference modulus of elasticity and simplified bending and shear design values.
3. Select Nominal beam size. The model converts nominal dimensions to common dressed dimensions before calculating section properties.
4. Enter Beam span (ft) as a positive decimal, such as 8 or 10.5. Enter Uniformly distributed load (lb/ft) as a positive total line load. Do not enter a floor area load in lb/ft² without first converting it to a line load using tributary width.
5. Select Deflection limit criteria. L/240 is less strict than L/360 or L/480. Read the live deflection, stress, utilization, and pass/fail results, then use Download Excel to save the current model.
6. Reset clears the demonstration and calculated content. Download Excel is then disabled until every required field has a complete valid value again.

Input guide

Wood species is required and must be one of the listed species groups; changing it changes stiffness and strength. Lumber grade is required; lower grades generally reduce capacity. Nominal beam size is required; increasing depth usually has a much larger effect than increasing width because the moment of inertia depends on depth cubed. Beam span (ft) accepts ordinary U.S. decimal notation from 0.1 to 100 ft; span strongly increases deflection because deflection varies with the fourth power of length. Uniformly distributed load (lb/ft) accepts 0.01 to 100,000 lb/ft; higher load increases deflection and both stresses linearly. Deflection limit criteria is required; a larger divisor means a smaller permitted deflection.

Output guide

Deflection due to loading is the estimated midspan movement in inches. Maximum allowable deflection is span divided by the selected criterion. Required bending stress is the extreme-fiber stress from the maximum simple-span moment. Adjusted bending value is the selected simplified reference capacity used here. Required shear stress is the average shear demand used by the reference-style model. Adjusted shear value is its comparison capacity. Governing utilization is the largest of deflection demand/limit, bending demand/capacity, and shear demand/capacity. A value below 100% passes this simplified screen; 100% or above fails.

Worked example

For the startup 2×10, the actual section is 1.5 in × 9.5 in. Its moment of inertia is 1.5 × 9.5³ ÷ 12 = 107.172 in⁴. With 240 lb/ft = 20 lb/in over 96 in and E = 1,900,000 psi, the deflection is 5wL⁴ ÷ (384EI) = about 0.109 in. The L/240 limit is 96 ÷ 240 = 0.400 in, so deflection passes. The table then compares bending and shear demand with the selected reference values and identifies the governing ratio.

Learn more

Review the American Wood Council's National Design Specification resources for formal wood design provisions, the Southern Forest Products Association's Southern Pine span tables for tabulated residential applications, and the USDA Forest Products Laboratory's Wood Handbook for engineering properties and wood behavior.