Deck beam span calculator
A beam's span is decided by how much deck leans on it, what it is cut from and how deep it is. Give it the joist span it carries and the member you want to build it from, and it reads the maximum span straight off IRC 2021 R507.5, Table R507.5(1), then lays out the posts in equal bays. Confirm the edition your jurisdiction has adopted before you dig.
40 psf live load and 10 psf dead load, No. 2 grade, no point loads. If your ground snow load is over 40 psf, Tables R507.5(2) to (4) govern and give shorter spans.
What it reads
Effective joist span. The beam table is not keyed by the joist span you measure. Table R507.5(5) turns it into an effective span: the actual joist span times a factor that depends on how far the joists cantilever past the beam. With no cantilever the factor is 0.66; at a cantilever of a quarter of the joist span it is 1.00. Between the listed ratios this tool takes the next larger one, which gives the shorter beam span.
The table row. Species group, member size and ply count pick the row of Table R507.5(1); the effective joist span picks the column, interpolated between the 6, 8, 10, 12, 14, 16 and 18-foot columns as the table's own note permits. Below 6 feet it uses the 6-foot column, which is the conservative side. Past 18 feet the table stops and so does the tool.
Plies and depth trade closely. In the table, southern pine over 12-foot joists with no cantilever: a 2-ply 2×8 spans 7'-7", a 3-ply 2×8 9'-6", a 2-ply 2×10 9'-0" and a 3-ply 2×10 11'-2". Run both ways before you buy: the cheaper beam is the one that saves a footing.
Worked example
A 12 × 16 deck in southern pine, joists spanning 12 feet onto one beam along the outer edge with no cantilever. Effective joist span is 12 × 0.66 = 7.9 feet. A 3-ply 2×10 may span 11'-2" there, so the 16-foot beam needs two bays: three posts, 8 feet apart. A 2-ply 2×8 may span only 7'-7", which needs three bays and four posts.
That is the trade worth running both ways: a bigger beam often costs less than the extra footing it saves, once you count the concrete and the digging. The same steps by hand, beam overhangs included, are in the deck post spacing calculator guide.
Where this stops
Point loads — a hot tub, a masonry planter, a post carrying a roof — are outside every prescriptive beam table, and outside this. So is a beam supporting a cantilever longer than the tables allow, and any beam on a deck more than one storey up.
Once the posts are placed, the load path continues down: each post needs a footing sized for its share, at your local depth.
Code requirement
Beam spans are prescribed in IRC 2021 R507.5. Your jurisdiction may amend the model code or adopt a different edition. Confirm with your building department before you cut.
Sources
- 2021 IRC § R507.5 and Tables R507.5(1)–(4), Deck Beams (UpCodes) read 2026-10-18
Filed under Deck framing & structure — 19 tools, guides and comparisons in this pillar.
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