#2 SPF lumber, simple span, L/360 LL deflection. Not engineered. Verify with a structural engineer, AWC span tables, or your local building department before purchase or framing.
About this calculator
This beam span calculator estimates the maximum simple-span length of a built-up dimensional-lumber beam (one to three plies of 2x6, 2x8, 2x10, or 2x12) based on its bending capacity and an L/360 live-load deflection limit. The calculation assumes #2 Spruce-Pine-Fir (Fb = 875 psi, E = 1,400,000 psi) with a size factor Cf, repetitive-member factor not applied (single-beam case), normal load duration. Enter the lumber size, the number of plies nailed or bolted into a built-up beam, the tributary width the beam is carrying, and the floor live and dead loads. Output is the maximum span where the beam stays within both bending strength and deflection limits. ESTIMATE ONLY — built-up beams should be sized by an engineer or a code-stamped span table for any actual structure.
Can I use this calculator to size a real load-bearing beam?
No — this is a planning estimator, not an engineering calculation. Real beam sizing must consider bearing length, point loads, lumber grade and species variation, repetitive vs single-member factors, snow/seismic loads, and field conditions the calculator cannot see. Use this to sanity-check whether a 2-2x10 versus 3-2x12 is in the right ballpark, then have an engineer or your local inspector confirm before purchase. Span tables from AWC (American Wood Council) and Weyerhaeuser are the next step up.
What does L/360 deflection mean?
L/360 is the maximum live-load deflection allowed at midspan, where L is the span length. For a 12-foot beam, L/360 = 144"/360 = 0.4" of sag under full live load. It is the standard for floor framing under finished ceilings (drywall cracks if it sags more). Roofs and decks use L/240 (less strict). This calculator uses L/360 as the deflection limit because most residential beams support floors.
What is tributary width for a beam?
Tributary width is the half-distance to the next parallel support on each side. If a beam runs down the middle of a 24-foot-wide room with joists landing on it from both sides, each side feeds 12 feet of joist span, so the tributary width is 12 feet. If the beam is at one wall and joists hang from a ledger on the opposite wall, the beam carries the full 24-foot width. Tributary width times floor load (psf) gives the uniform load (plf) on the beam.