📐 Beam Dimensions & Wood Species
in
Enter beam width in inches (e.g. 4 for a 4x beam).
Nominal width: 2, 3, 4, 6, 8 in — actual is 0.5 in less
in
Enter beam depth in inches (e.g. 12 for a x12 beam).
Nominal depth: 6, 8, 10, 12, 14, 16 in
ft
Enter the span in feet (1–60 ft).
Clear distance between supports
Select species/grade — affects Fb and E values
plf
Enter total load in pounds per linear foot (plf).
Dead + Live load. Residential floor: 400–600 plf typical
L/360 is the IRC standard for residential floors
Maximum Safe Span
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⚠️ Engineering Disclaimer: This calculator provides preliminary estimates based on simplified beam formulas and published allowable stress values. It does not account for lateral buckling, load duration factors, wet service conditions, notching, or connection details. All structural work must be reviewed and approved by a licensed structural engineer. Never use this tool as the sole basis for construction decisions.
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Sources & Methodology
Beam formulas and allowable stress values verified against the 2024 National Design Specification (NDS) for Wood Construction and the 2024 International Residential Code (IRC) span tables.
AWC — 2024 National Design Specification (NDS) for Wood Construction
Primary authority for allowable bending stress (Fb), modulus of elasticity (E), and section property values for sawn lumber and engineered wood products.
ICC — 2024 International Residential Code (IRC) — Chapter 5 Floors
IRC span tables for floor joists and beams. L/360 deflection limit standard for live loads on floors (Section R301.7) used as default in this calculator.
WoodWorks — Structural Timber Span Tables & Design Resources
Published beam span tables and species allowable stress data verified against NDS and used as cross-reference for species Fb and E values.
Methodology:
Section Modulus: S = b x d^2 / 6 | Moment of Inertia: I = b x d^3 / 12
Max Moment (uniform load): M = w x L^2 / 8 | Required S: S_req = M / Fb
Max Deflection: delta = 5 x w x L^4 / (384 x E x I)
Max span from bending: L_bend = sqrt(8 x Fb x S / w). Max span from deflection: L_defl = (384 x E x I x delta_limit / (5 x w))^(1/3). Governing span = minimum of L_bend and L_defl. Units: w in lb/in, L in inches, Fb in psi, E in psi. All dimensions entered as nominal inches; calculation uses nominal values as input (actual sizes handled by the user noting the nominal vs actual convention).
Last reviewed: April 2026 — NDS 2024 values
How Is Wood Beam Span Calculated?
Wood beam span calculations involve two separate checks that must both be satisfied: a bending strength check (the beam must not break) and a deflection check (the beam must not sag too much). The governing span is whichever limit is reached first. For most residential applications, deflection controls the design more than bending strength.
M = w x L^2 / 8 (maximum moment, uniform load)
S = b x d^2 / 6 (section modulus of rectangular beam)
Worked Example — 4x12 Douglas Fir No.2, 400 plf, L/360 limit:
b = 4 in, d = 12 in. S = 4 × 144 / 6 = 96 in³. I = 4 × 1728 / 12 = 576 in&sup4;
Fb = 1,200 psi. Max moment capacity: M = Fb × S = 1,200 × 96 = 115,200 lb·in
w = 400 plf = 33.33 lb/in. L from bending = sqrt(8 × 115,200 / 33.33) = sqrt(27,659) = 166 in = 13.8 ft
E = 1,600,000 psi. L/360 limit: delta_allow = L/360. Solving for L_defl ≈ 12.4 ft
Governing span (deflection controls) = 12.4 ft
b = 4 in, d = 12 in. S = 4 × 144 / 6 = 96 in³. I = 4 × 1728 / 12 = 576 in&sup4;
Fb = 1,200 psi. Max moment capacity: M = Fb × S = 1,200 × 96 = 115,200 lb·in
w = 400 plf = 33.33 lb/in. L from bending = sqrt(8 × 115,200 / 33.33) = sqrt(27,659) = 166 in = 13.8 ft
E = 1,600,000 psi. L/360 limit: delta_allow = L/360. Solving for L_defl ≈ 12.4 ft
Governing span (deflection controls) = 12.4 ft
Allowable Stress Values by Species (2026 NDS)
| Species & Grade | Fb (psi) | E (psi) | Best For |
|---|---|---|---|
| Douglas Fir-Larch No.2 | 900–1,200 | 1,600,000 | General framing, beams |
| Southern Yellow Pine No.2 | 1,050 | 1,600,000 | Floors, southern U.S. |
| Hem-Fir No.2 | 850 | 1,300,000 | Western U.S. construction |
| Spruce-Pine-Fir No.2 | 875 | 1,400,000 | General use, widely available |
| LVL 2.0E Engineered | 2,600–3,100 | 1,900,000–2,000,000 | Long spans, headers, ridge beams |
L/360 vs L/240 vs L/480 Deflection Limits
The deflection limit is expressed as a fraction of the span. L/360 means the beam can deflect at most 1 inch for every 360 inches (30 feet) of span. The IRC requires L/360 for live loads on floors, meaning a 15-foot floor beam can deflect no more than 0.5 inches. L/240 applies to roofs and less critical applications. L/480 is specified for tile or stone floors where excessive deflection would crack grout or tile joints.
When to Specify LVL vs Solid Sawn Lumber
LVL (Laminated Veneer Lumber) becomes the economical choice when solid sawn lumber cannot achieve the required span without oversizing. With Fb values of 2,600 to 3,100 psi compared to 850 to 1,200 psi for common sawn lumber, an LVL can achieve spans 50 to 70% longer than the equivalent sawn lumber size. LVL is also dimensionally stable, virtually free of warping, and available in longer lengths. For spans over 14 feet, LVL or parallel strand lumber (PSL) is almost always specified.
Tributary Width and Load Calculation
The total load on a beam equals the tributary width times the total floor or roof load per square foot. Tributary width is the area of floor the beam supports on each side (half the joist span on each side). For a beam in the center of a room with 10-foot joists on each side: tributary width = 10 ft. With a total floor load of 50 psf (40 live + 10 dead): beam load = 10 ft × 50 psf = 500 pounds per linear foot (plf).
💡 Engineering Requirement: This calculator provides a preliminary estimate only. Any structural beam in a residence or commercial building must be reviewed by a licensed structural engineer, especially when removing load-bearing walls, installing ridge beams, or spanning openings over 6 feet. Many jurisdictions require stamped engineering drawings for permit applications involving structural beam work.
Frequently Asked Questions
Beam span depends on size, species, and load. A 4x12 Douglas Fir No.2 beam spans roughly 12 to 14 feet at 400 plf total load with L/360 deflection limit. A 6x12 at the same conditions spans about 15 to 17 feet. LVL beams can span significantly further. Use the calculator above to find the maximum span for your specific beam size and loading.
L/360 means the beam can deflect no more than the span divided by 360. For a 12-foot (144 inch) span: maximum allowable deflection = 144/360 = 0.40 inches. The IRC requires L/360 for live loads on floors. For total load deflection, L/240 is commonly used. L/480 is specified for tile floors to prevent cracking grout joints.
Fb is the allowable bending stress in psi from the NDS design values. Common values: Douglas Fir-Larch No.2 = 900 to 1,200 psi, Southern Yellow Pine No.2 = 1,050 psi, Hem-Fir No.2 = 850 psi. LVL has much higher values of 2,600 to 3,100 psi. Higher Fb allows longer spans or lighter beams for the same load.
Calculate total load (dead + live in plf), find required section modulus using S_req = M/Fb where M = wL^2/8, then select a beam where actual S is greater than or equal to S_req. Then verify deflection with delta = 5wL^4/(384EI) is within your L/360 or L/240 limit. The calculator above performs both checks automatically.
A header spans a door or window opening and carries the load from the structure above. A floor beam (girder) supports floor joists and carries the weight of the floor system. Headers are sized based on opening width and stories above. Floor beams must support tributary area times total floor load (typically 50 psf for living areas: 40 psf live + 10 psf dead).
LVL (Laminated Veneer Lumber) is engineered wood with Fb values of 2,600 to 3,100 psi compared to 850 to 1,200 psi for solid sawn lumber. This allows spans 50 to 70% longer than equivalent sawn lumber. LVL is dimensionally stable and available in long lengths. It becomes economical for spans over 14 feet where solid lumber would require very deep sections.
Yes, for any structural application. This calculator provides preliminary estimates only. Licensed structural engineers are required by most jurisdictions for load-bearing wall removals, ridge beam installations, and any opening over 6 feet in many areas. Engineering stamps are often required for building permits involving structural beam work. Never rely solely on a calculator for structural decisions.
A 4x6 Douglas Fir No.2 beam has a section modulus of 17.6 in^3. At 200 plf total load, it can span roughly 8 to 9 feet before exceeding deflection limits. At 400 plf, it is adequate for only about 6 to 7 feet. For heavier loads or longer spans, step up to a 4x10 or 4x12. Use the calculator above for your specific combination.
Tributary width is the floor area the beam supports on each side, measured perpendicular to the beam. For a center beam with 10-foot joists on each side, tributary width = 10 feet. Total load per linear foot of beam = tributary width times total floor load psf. Example: 10 ft x 50 psf = 500 plf on the beam.
Modulus of elasticity (E) measures a wood species resistance to deformation under load, in psi. Higher E means less deflection for the same beam and load. Douglas Fir and Southern Yellow Pine have E = 1,600,000 psi. LVL has E = 1,800,000 to 2,000,000 psi. E is the key factor in deflection calculations; it does not affect bending strength directly.
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