Deck beam sizer
Enter the joist span, beam material and post spacing, and see which 1, 2 or 3-ply beam the IRC allows (Tables R507.5).
Result
No cantilever means the ledger carries its share — the table credits your beam a third off the joist span.
How is this calculated?
Using the tool
The beam is the deck's backbone: it collects everything the joists carry and hands it to the posts. Tell it how far your joists span (and hang over), what the beam is made of, and how far apart the posts land — and read which 1, 2 or 3-ply build the IRC allows, straight from Tables R507.5(1)–(4) with the effective-span factor applied the way the code intends.
The code tables
A deck beam is sized by how much deck leans on it, and the table measures that with the joist span: longer joists mean a wider strip of deck lands on every foot of beam. That's why each column of Table R507.5 is a joist span, and the beam span shrinks as you read right.
The effective span factoris the honest part: joists that simply rest between ledger and beam split their load — the ledger takes a share — so the code lets you enter the table at 0.66 × the joist span. Hang the joists past the beam and the lever arm starts working against you: the factor climbs with the cantilever until, at a quarter of the span, the full joist length counts. Plies work like lanes on a bridge — two 2×10s carry roughly double one — while depth buys more than width, which is why a 2×12 beats adding a ply more often than you'd guess.