Door Header Size Calculator

Use this door header size calculator to find the correct lumber size for your door header or determine the maximum length that a piece of lumber for a door header can span.

Allowable span9 ft 0.51 in9.042 ft — bending stress governs

Limited by strength9 ft 0.51 in9.042 ft at 1,200 psi allowable

Limited by deflection9 ft 11.52 in9.96 ft at L/360

What governsbending stressin timber it is usually deflection

Beam section7 × 7.25 in2 plies

Section modulus61.323 in³

Second moment of area222.296 in⁴

Total load at that span5,425 lb

Load at a 10 ft span490.6 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.