Deck beam span calculator
A beam's span is decided by how much deck leans on it and how deep it is. Give it the joist span it carries and the member you want to build it from, and it lays out the posts. Treat the answer as a layout, then confirm the number against IRC 2021 R507.5 as adopted where you build.
A planning estimate for laying out footings. Confirm the actual span against the beam table your jurisdiction has adopted before you buy or dig.
What it counts
Tributary width is the strip of deck the beam actually holds up: half the joist span on each side of it. A beam catching one side of a 12-foot joist span carries a 6-foot strip.
Depth beats plies. A beam's bending capacity rises with the square of its depth but only linearly with the number of plies. Going from 2×8 to 2×10 is worth more than adding a third 2×8, and it usually costs less.
The output is a post layout for planning footings. It is deliberately conservative and it is not a substitute for the adopted table, which accounts for species, grade and your local loads.
Worked example
A 12 × 16 deck, joists spanning 12 feet onto one beam along the outer edge. Tributary width is 6 feet. A 3-ply 2×10 gives a much longer post spacing than a 2-ply 2×8 over the same width — which usually means three posts instead of four, and one less hole to dig, inspect and fill.
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.
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 2026-09-04