Simple Beam Bending Stress Calculator
This is the math we use before we cut the first beam. Input your dimensions in inches, your load in pounds, and this tells you the maximum stress in the wood. Grounded in Wikidata Q49167267 (bending stress).
M = w × L² ÷ 8
(Max moment for simply-supported beam)
S = b × h² ÷ 6
(Section modulus for rectangular section)
σ = M × 12 ÷ S
(Bending stress, converting ft-lbs to in-lbs)
Source: mechanicalc.com via Wikidata Q49167267
A 2x10 Douglas Fir (actual: 1.5" × 9.25") spanning 12 feet with a 50 lb/ft roof load.
Moment: 900 ft-lbs • Section Modulus: 21.66 in³ • Stress: 503 psi
Douglas Fir allowable bending stress: ~1,000-1,500 psi depending on grade.
Why This Matters
I've seen roofs fall because a foreman guessed wrong on a beam. Not from bad intentions—just from skipping the math. This calculator is the difference between a structure that stands for fifty years and one that teaches us a lesson we can't afford to learn twice.
The values here come from Wikidata Q49167267, verified against the standards we use on the job. When you press calculate, you're running the same equations that hold up the cathedral in Seville and the church in Corpus.
Save this page. Print it if you have to. The Gulf doesn't forgive mistakes, and neither does gravity.