Deck Beam Span & Post Spacing Calculator (IRC R507.5)
Determine the maximum deck beam span (post spacing) for built-up 2-ply and 3-ply beams from the IRC beam span table based on the joist span it carries, check your planned post spacing, find the smallest beam that works and the number of posts needed.
How it works
Beam span from IRC Table R507.5 (beam size × joist span carried); cantilever ≤ beam span ÷ 4; posts = ceil(length ÷ spacing) + 1
A deck beam collects the load from every joist it carries, so its allowable span shrinks as the joists get longer — a 2-ply 2×10 can run over 10 ft between posts under 6 ft joists but only about 7 ft under 12 ft joists. The IRC table encodes that trade-off for built-up dimensional lumber beams; three plies buy roughly 25% more span than two, and going up a size buys 15–20%. Because posts and footings are cheap relative to a bigger beam, most decks use more posts rather than the largest beam, and cantilevering the beam a quarter-span past the end posts reduces the span in between. The code is strict about how the beam sits on the post: on top, bearing fully, never hung on bolts through the side.
Worked example
A 2-ply 2×10 southern pine beam carrying 12 ft joists may span 7′-4″, so posts planned at 8 ft are 8 inches too far apart: use a 3-ply 2×10 (9′-2″), a 2-ply 2×12 (8′-7″), or three spans of 5′-4″ with four posts on a 16 ft beam.
Frequently asked questions
How do I account for joist cantilevers when picking the beam row?
Use the joist span plus half the overhang: a 10 ft span with a 2 ft cantilever loads the beam like an 11 ft span (some jurisdictions say use the full overhang — check).
Can I splice the beam plies?
Yes, but butt joints must fall over a post, staggered between plies, with the plies nailed or screwed together per R507.5. A single 2× ply is never a beam.
Related calculators
Span values follow the IRC 2021 deck provisions (R507) and AWC DCA 6 for #2 grade lumber at 40 psf live + 10 psf dead load, wet-service, simplified to the common cases. Local amendments, snow loads, hot tubs, cantilevers and other species/grades change the numbers — confirm with the local building department or an engineer.