Wood Beam Span Calculator

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Species group determines the reference design values (Fb, Fv, E) from the NDS Supplement.
Higher grades have larger allowable stress values. No. 2 is the most common framing grade.
Nominal width in inches. Actual dressed dimension is 0.5" less.
Nominal depth in inches. Deeper beams resist bending far more effectively.
Clear distance between supports (center-to-center of bearing). The most critical dimension for deflection.
ft
Total load per foot of beam length including dead and live loads. A typical floor beam carries 40-60 psf over its tributary width.
lb/ft
Allowable deflection as a fraction of span. L/360 is the most common requirement for floor beams.
Wood can carry higher stresses for shorter durations. The CD factor adjusts Fb and Fv for this.
Lumber used in wet conditions loses strength. The wet service factor CM reduces allowable values.
Actual deflectionPasses NDS checks
0.213

Midspan deflection under the applied load

Allowable deflection0.4
Deflection utilization0.5%
Required bending stress (fb)866psi
Allowable bending stress (Fb')900psi
Bending utilization1%
Required shear stress (fv)55.6psi
Allowable shear stress (Fv')180psi
Shear utilization0.3%
Overall resultPASS - all three checks satisfied
Section modulus (S)49.91in³
Moment of inertia (I)230.84in⁴
1% ratio
Good margin<0.75Near limit0.75-1Over limit1+
Deflection0.5%
Bending1%
Shear0.3%

This 4x10 Douglas Fir-Larch beam passes all NDS checks.

  • The beam is at 96% of its controlling limit - it has 4% reserve capacity.
  • Using Douglas Fir-Larch, No. 2 grade, 4"x10" nominal.
  • This calculator uses simplified NDS formulas for a uniformly loaded simple span. For point loads, cantilevers, or notched beams consult a licensed structural engineer.

Next stepVerify the bearing length at each support and check that the beam is adequately braced against lateral buckling (or reduce the beam stability factor Cl).

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