Wood Beam Design Calculator

Check bending adequacy of wood beams per NDS ASD: compare actual bending stress against adjusted allowable stress.

Inputs

Span Length

Uniform Load

kN/m

Species Adjustment Factor

NDS Supplement Table 4A: DF-L=1.0 reference; Southern Pine higher Fb; SPF lower; adjust relative

Grade Adjustment Factor

Beam Width

Beam Depth

Moisture Factor (CM)

NDS Table 4A: dry use CM=1.0; wet service CM=0.85 (Fb), 0.97 (E), 0.67 (Fv)

Temperature Factor (Ct)

Tab to move between fields

Results

Maximum Moment

10 kN·m

Actual Bending Stress (fb)

12.21 MPa

Adequate?

0 (1=yes, 0=no)

Required Bending Stress12.21 MPa

Adjusted Allowable (F'b)10 MPa

How to Use This Calculator

Enter the beam span in feet and the tributary width.
Input the dead load, live load, and any point loads in lbs or psf.
Set the wood species, grade, and moisture condition.
Review the Required Section Modulus, Recommended Lumber Size, and the Deflection Check.
Apply size adjustment factors and verify shear capacity at the supports.

How the result changes with Span Length

Span Length	Maximum Moment	Actual Bending Stress (fb)	Adequate?
1.95	2.38 kN·m	2.9 MPa	1 (1=yes, 0=no)
5.57	19.39 kN·m	23.67 MPa	0 (1=yes, 0=no)
9.93	61.63 kN·m	75.23 MPa	0 (1=yes, 0=no)
14	122.5 kN·m	149.54 MPa	0 (1=yes, 0=no)

What each input means

Span Length: Clear span length of the wood beam between supports.
Uniform Load: Total uniformly distributed load on the beam.
Species Adjustment Factor: Species-dependent adjustment per NDS Supplement Table 4A. Douglas Fir-Larch (visually graded sawn): Fb base 1,000–1,500 psi; Southern Pine: Fb up to 1,850 psi; Spruce-Pine-Fir (SPF): Fb 875–1,150 psi. Factor adjusts Fb relative to DF-L.
Grade Adjustment Factor: Lumber grade factor. Select = 1.0, No.1 = 0.86, No.2 = 0.72.
Beam Width: Actual width of the beam cross-section (e.g., 2×10 actual = 38 mm, 4×10 actual = 89 mm).
Beam Depth: Actual depth of the beam cross-section (e.g., 2×10 actual depth = 235 mm).
Moisture Factor (CM): Wet service factor per NDS Table 4A adjustment factor CM. 1.0 for dry conditions (MC ≤ 19% for sawn lumber, per NDS §4.1.4); CM=0.85 for Fb; CM=0.97 for E; CM=0.67 for Fv in wet service (MC>19%).
Temperature Factor (Ct): Temperature factor. 1.0 for T ≤ 100°F, 0.9 for 100-125°F, 0.7 for 125-150°F.

How this is calculated

Worked example, using the default values

Identify Input Parameters
4 parameters
Span Length = 4, Uniform Load = 5, Species Adjustment Factor = 1, Grade Adjustment Factor = 1 = 8 input(s) provided
Calculate Maximum Moment
Maximum Moment
10 = 10
Calculate Actual Bending Stress
Actual Bending Stress
12.21 = 12.21
Calculate Adequate?
Adequate?
0 = 0
Calculate Required Bending Stress
Required Bending Stress
12.21 = 12.21
Calculate Adjusted Allowable
Adjusted Allowable
10 = 10

Engine last updated July 27, 2026. Checked against 1 independently-derived test — how we verify calculators.

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