Wood Beam Span Calculator

Use this wood beam span calculator to check a wood beam of a particular size and length to see if it can support a given uniform linear load by comparing its allowable and required bending and shear stress.

Allowable span9 ft 11.89 in9.991 ft — bending stress governs

Limited by strength9 ft 11.89 in9.991 ft at 1,200 psi allowable

Limited by deflection11 ft 6.54 in11.545 ft at L/360

What governsbending stressin timber it is usually deflection

Beam section3.5 × 9.25 insingle ply

Section modulus49.911 in³

Second moment of area230.84 in⁴

Total load at that span3,996 lb

Load at a 10 ft span399.3 lb/ftwhat this section could carry over 10 ft, by strength

MaterialDouglas fir–larchE = 1,800,000 psi

The formula

span from the allowable bending stress: L = √(8 S Fb ÷ w)

Deflection usually wins

Timber is strong relative to its stiffness, so a beam sized to a deflection limit is almost always the binding constraint. That is why the two answers here differ, and why the smaller one is the one to build to.

These are ideal-case numbers using nominal allowable stresses for a simply supported beam with a uniform load. Real design applies adjustment factors for load duration, wet service, member size, repetitive use and more, and it also checks shear and bearing. Use this to sanity-check a proposal, and have a header or beam properly designed before you build it.