Structural Beam Sizing Calculator

Find the minimum beam size that passes NDS bending, shear, and deflection checks — for any span, load, and wood species.

Beam Parameters

Beam Application

Beam Span (ft)

Tributary Width (ft)

Half the distance to the next beam or bearing wall on each side

Loads (psf)

Dead Load

psf

Live Load

psf

Floor: DL 10–15 psf, LL 40 psf residential

Material & Grade

Beam Type

Species

Grade

Load Duration Factor (C_D)

Minimum Beam Required

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Bending (fb ≤ Fb)

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Shear (fv ≤ Fv)

—

Live Load Deflection (≤ L/360)

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Total Deflection (≤ L/240)

—

Uniform Load (plf)

—

Max Moment (ft·lbf)

—

Live Deflection

—

Total Deflection

Beam Diagram (Uniform Load)

IRC Code References

IRC R602.7 Headers for load-bearing walls — specifies minimum header sizes for openings based on span and supported loads.

IRC Table R502.5 Girder spans for joists framing from one side — use for floor beam spans supporting single-span joists.

IRC R802.4 Ridge beam sizing for roofs — ridge beams carrying vertical load must be sized per this section, not as a ridge board.

NDS ASD This calculator uses NDS Allowable Stress Design (bending fb ≤ Fb, shear fv ≤ Fv, deflection L/360 live / L/240 total).

Always verify beam sizing with your local building code and a licensed structural engineer before construction. Local amendments may supersede IRC defaults.

Beam Size Comparison

All available beam sizes for the current load condition — green passes all checks, red fails one or more.

Deflection vs. Span

Live load deflection for the selected beam size across a range of spans. The dashed line shows the L/360 limit.

Utilization Ratios

How close each check is to its limit (100% = at limit, <100% = passes with margin).

NDS Design Values — Sawn Lumber (Posts & Timbers)

Reference design values from NDS Supplement Table 4D. Values in psi. C_D=1.0, C_M=1.0 (dry service).

Deflection Limits (IBC / IRC)

Floor joists/beams: L/360 live, L/240 total
Roof rafters (no finish ceiling): L/180 live
Roof rafters (with finish ceiling): L/240 live
Headers: L/360 live, L/240 total

Duration Factors (C_D) — NDS 2.3.2

Permanent load: C_D = 0.9
Ten-year load (snow): C_D = 1.0
Two-month (construction): C_D = 1.15
Seven-day (roof live): C_D = 1.25
Wind / seismic: C_D = 1.6

Glulam Design Values

24F-V4 (western species): F_b = 2400 psi
24F-V8 (southern pine): F_b = 2400 psi
E_x = 1,800,000 psi (most 24F grades)
F_v = 265 psi (western), 300 psi (SP)

LVL Design Values (Typical)

F_b = 2,600 psi (1.75" × up to 18" deep)
E = 1,900,000 psi (Microllam 2.0E)
F_v = 285 psi
Verify actual values with product manufacturer's ESR

Disclaimer: This calculator uses simplified NDS allowable stress design values for preliminary sizing. Always verify final beam sizes with a licensed structural engineer, especially for unusual loads, long spans, or complex structural conditions.

How to Size a Beam

Determine the span — Measure center-to-center between supports (posts, walls, or columns).

Find the tributary width — This is half the distance to the next beam on each side. A 20 ft room with a center beam has a 10 ft tributary width per side.

Set your loads — Use 10–15 psf dead load for floors, 40 psf live load for residential floors (IRC Table R301.5). Decks use 40 psf LL. Roofs use 20 psf snow or per your climate zone.

Choose the species and grade — Douglas Fir #1 or Southern Pine are the strongest common options. #2 saves money but requires a larger beam.

Read the result — The recommended beam is the smallest size that passes all four checks: bending stress, shear stress, live load deflection, and total deflection.

Verify with a pro — This calculator is for preliminary sizing and estimation. Licensed engineer review is required for permitted projects.

Beam Design Terminology

F_b (Allowable Bending Stress)The maximum bending stress the wood species/grade can sustain. Adjusted by CD, CM, CF, etc.

F_v (Allowable Shear Stress)Maximum horizontal shear stress. Sawn lumber shear governs for short beams with high loads.

S (Section Modulus)A geometric property = bd²/6 for rectangular sections. Larger S → more bending resistance.

I (Moment of Inertia)A geometric property = bd³/12. Controls deflection — the stiffer check for longer spans.

C_D (Duration Factor)Wood resists higher stresses for shorter loading durations. CD = 1.0 for 10-year (floor), 1.15 for construction loads.

Tributary WidthThe plan area width each beam supports. A center beam in a 20 ft span room carries 10 ft of tributary width per side.

Glulam (Glued Laminated Timber)Engineered beams with higher allowable stresses than sawn lumber. Common in spans > 16 ft.

LVL (Laminated Veneer Lumber)Engineered product with uniform properties and high strength. Common for headers and long-span beams.

Frequently Asked Questions

What is the difference between a floor beam, deck beam, and ridge beam?

A floor beam carries the combined dead load (floor framing, subfloor, finishes) and live load (occupancy). A deck beam carries deck dead load and 40 psf live load per code. A ridge beam supports roof load from both sides and must be sized for that full tributary width — it is not the same as a ridge board, which does not carry vertical load.

Why does deflection often govern instead of bending stress?

For longer spans (typically > 10–14 ft), the beam becomes stiff enough to handle the bending stress but still deflects too much. This is because deflection grows with the fourth power of span (L⁴), while bending moment only grows with the square (L²). The L/360 deflection limit is often what drives the beam selection for moderate spans.

Can I use two 2×12s instead of a 4×12 or 6×12 beam?

Yes — doubled or tripled 2× members are common and often stronger than single solid timber of the same net cross-section. Two 2×12s (each 1.5" wide) give a combined b = 3.0", which is slightly less than a 4×12 (b = 3.5"). However, they're easier to handle and are code-accepted when properly nailed. This calculator shows solid beam sizes; multiply nominal width by the number of plies for built-up beams.

When should I use glulam or LVL instead of sawn lumber?

Glulam and LVL have higher allowable bending stress (2400–2600 psi vs. 900–1200 psi for sawn lumber) and fewer defects. They're the preferred choice for spans longer than 16–20 ft, for open-plan floors where a steel post or column isn't an option, and anywhere appearance matters. They also handle moisture cycling better than solid sawn timber for exposed outdoor applications.

Do I need a structural engineer for a beam I size here?

For permitted residential construction, yes — a licensed structural engineer or architect must stamp the plans in most jurisdictions. This calculator is excellent for preliminary sizing and estimating lumber costs, but final beam specifications must be verified by a professional who considers your specific load path, connections, bearing conditions, and local code requirements.