Wood Beam Calculator: Joists, Headers, Glulam & LVL (NDS)

Q: What is the load duration factor CD?

Wood carries higher loads for short durations. NDS uses CD = 0.9 for permanent dead load, 1.0 for occupancy live load, 1.15 for snow, 1.25 for roof live load and 1.6 for wind. Each load combination uses the factor for its shortest-duration load, so a dead-load-only combination can govern even though its load is smaller.

1 · Member

Product

Species

Grade

Size

Plies

Repetitive member (joists or rafters at ≤ 24 in. o.c. with sheathing, or 3+ plies), C_r = 1.15

Stress class

Width b in.

Depth d in.

Grade

Depth in.

Plies (1-3/4 in. each)

Generic LVL values vary by manufacturer: confirm with the product's evaluation report.

Bearing length in.

Wet service (C_M)

Include creep in total deflection (K_cr on dead load)

2 · Spans and supports

Support	Type	Span to next (ft)

Pinned supports also act as rollers. Use a free end for a cantilever or overhang. Up to 5 spans.

3 · Service loads Include self-weight in D

Distributed loads (plf, downward positive)

Case	w start	w end	From (ft)	To (ft)

Leave From and To blank to load the full length. For joists, w = pressure (psf) × spacing (ft).

Point loads (lb, downward positive)

Case	P	x (ft)

x is measured from the left end. D dead, L live, L_r roof live, S snow, W wind (enter uplift as negative).

Pattern live load L on continuous spans (ASCE 7-22 §4.3.3)

4 · Lateral support and deflection

Compression edge bracing

Brace spacing ft

Brace points ft from left

Top edge continuously braced (positive moment needs no stability reduction)

Live deflection limit L/

Total deflection limit L/

Cantilevers and overhangs use twice their length in the deflection limit.

How the wood beam calculator works

This free wood beam calculator sizes and checks floor joists, rafters, headers and beams in sawn lumber, glued laminated timber (glulam) and laminated veneer lumber (LVL) by the 2018 National Design Specification for Wood Construction (NDS), the edition referenced by the Florida Building Code, 8th Edition (2023). Choose the species and grade or product, the size and number of plies, the spans and supports, and the dead, live, roof live, snow and wind loads. The calculator applies every adjustment factor, checks each ASCE 7-22 load combination and draws the shear, moment and deflection diagrams.

What it checks

Bending stress f_b = M/S against F_b′ with the load duration factor C_D, wet service C_M, size factor C_F, repetitive member factor C_r, glulam volume factor C_V and the beam stability factor C_L from the unbraced length
Shear stress f_v = 1.5V/bd against F_v′
Bearing at each support, f_c⊥ = R/(b·l_b) against F_c⊥′
Live and total load deflection, with creep (K_cr = 1.5 dry, 2.0 wet) applied to the dead load in the total deflection
Glulam negative moment over supports and cantilevers with F_bx⁻

Method

In ASD, the load duration factor for each combination is set by its shortest-duration load: 0.9 for dead load alone, 1.0 with live load, 1.15 with snow, 1.25 with roof live load and 1.6 with wind. In LRFD, the reference values are multiplied by the format conversion factor K_F, the resistance factor φ and the time effect factor λ of NDS Appendix N. The beam is analyzed with the same stiffness-method engine as the Static Structures steel beam calculator, so continuous joists, cantilevers and overhangs are handled with pattern live loading.

Assumptions and limitations

Solid rectangular members bending about the strong axis at normal temperatures, without notches, holes, tapers or incising.
Reference design values follow the NDS 2018 Supplement for sawn lumber and glulam; the LVL values are generic and must be replaced with the manufacturer's published values.
Shear is checked at the maximum shear without the reduction for loads within d of the support, which is conservative.
Built-up members must be fastened so the plies act together.

Frequently asked questions

What size wood beam do I need?

Enter the span and the loads it carries, then try sizes until bending, shear, bearing and deflection all pass. For a header, the load is usually the floor or roof load times half the span of the joists or rafters it supports.

What is the load duration factor C_D?

Wood carries higher loads for short durations. NDS uses C_D = 0.9 for permanent dead load, 1.0 for occupancy live load, 1.15 for snow, 1.25 for roof live load and 1.6 for wind. Each load combination uses the factor for its shortest-duration load, so a dead-load-only combination can govern even though its load is smaller.

When does the beam stability factor C_L apply?

When the compression edge is not braced along its length. Floor sheathing braces joists, so C_L = 1.0. Headers and beams braced only at the supports get a reduction based on the effective unbraced length, depth and width. Sawn lumber with a depth-to-width ratio of 2 or less needs no lateral support.

Should I choose Southern Pine or Douglas Fir?

Use the species available where you build. Southern Pine values in NDS Table 4B already include the size effect and vary by depth, while Douglas Fir-Larch, Hem-Fir and SPF use Table 4A values with the size factor C_F.

Related calculators

For educational and preliminary design purposes only. Always verify results with a licensed PE.