Deck Beam Span Calculator
Size a built-up deck beam from the IRC R507.5 table. Pick the species and the deck joist span it carries to get the maximum span for double and triple 2× beams and the minimum size for your post spacing. Free, no sign-up.
What to calculate next
Tools commonly used alongside this calculation
Joist Span Calculator
Find the maximum allowable floor or deck joist span from the IRC span tables. Pick species, grade, size, and spacing to get the code max span by joist size and check it against the span you need. Free, no sign-up.
Rafter Span Calculator
Find the maximum horizontal roof rafter span from the IRC R802.4.1 tables. Pick the roof or ground snow load, species, grade, size, and spacing to get the max span by size and the minimum rafter for your roof. Free, no sign-up.
Stair Stringer Calculator
Enter the total rise to get the number of risers, the riser height, tread count, total run, and stringer length, with an IRC R311.7 code check on riser and tread. Free, no sign-up.
Explanation
A deck beam carries the deck joists and transfers their load down to the posts and footings. How far that beam can span between posts depends on three things: the beam size, the lumber species, and how much floor the beam is carrying — which is set by the deck joist span it supports. The International Residential Code (IRC) gives prescriptive beam spans in Table R507.5 so you don't have to run the numbers. This calculator reads that table: choose the species and the joist span the beam carries, and it returns the maximum span for each built-up beam size and the smallest one that reaches your posts.
How the R507.5 beam table works
Deck beams are usually built up from two or three 2× boards nailed together — a “(2) 2×10” is two 2×10s, a “(3) 2×10” is three. More plies and more depth span farther. The other axis is the supported joist span: the longer the joists, the more load lands on the beam, so its allowable span shrinks. Read the table at the joist-span column nearest your deck (round up if you fall between, since interpolation only lengthens the span). The example below is Southern Pine carrying a 12 ft joist span.
| Built-up beam (SP, 12 ft joists) | Max beam span |
|---|---|
| (2) 2×6 | 4'-10" |
| (2) 2×8 | 6'-2" |
| (2) 2×10 | 7'-4" |
| (2) 2×12 | 8'-7" |
| (3) 2×6 | 6'-1" |
| (3) 2×8 | 7'-9" |
| (3) 2×10 | 9'-2" |
| (3) 2×12 | 10'-9" |
Example: IRC Table R507.5(1), Southern Pine, 40 psf live load, beam carrying a 12 ft deck joist span.
Sizing the whole deck
The beam is one link in the load path. Start by sizing the joists for their span and spacing with the joist span calculator, then bring that joist span here to size the beam and set your post spacing. The posts then bear on concrete footings — size the piers and estimate the pour with the concrete calculator. Beam plies must be fastened with two rows of 10d nails at 16 in on-center, and splices have to land over a post so each ply bears fully.
Notes and limitations
This table is for built-up beams of two or three 2× boards at a 40 psf live load with no snow. A single 2× member used as a beam on a small porch or landing, solid 4× timbers, and decks in snow country (the R507.5 snow tables) fall outside this tool — size those from the full IRC table or the AWC DCA6 deck guide. Beam cantilevers past a post are limited to the beam span divided by four.
This calculator reads the IRC R507.5(1) built-up beam table and flags the smallest beam that reaches your posts. It does not design the posts, footings, ledger, or connections, account for snow or concentrated loads, or replace a stamped plan. Confirm the governing code edition and any local amendments with the authority having jurisdiction.