Deck joist span calculator
Pick the species group you are buying, the spacing you are framing at, and the clear span the joists have to cross. The list below shows what every size in IRC 2021 R507.6 reaches, and marks the smallest one that carries it. Clear span is face of ledger to face of beam — not the length of the board.
What the table actually counts
The span in the code table is the clear horizontal distance the joist crosses unsupported. A 16-foot board sitting on a ledger at one end and a beam 13 feet away has a 13-foot span; the three feet hanging past the beam is cantilever, and the table above assumes there is none. Add a cantilever and the allowed back-span changes.
Species matters more than most people expect. At 16-inch centres a 2×10 carries 14'-0" in southern pine and 13'-0" in western cedar — the same stick of lumber by name, a foot of difference in what it will hold. Buy for the span, not for the label.
Code requirement
Your jurisdiction may amend the model code or adopt a different edition. Confirm with your building department before you cut.
Worked example
A 12 × 16 deck off the back wall, ledger on the house, one beam on posts. Run the joists the 12-foot direction and the clear span is about 11'-6" once the beam sits inboard. At 16-inch centres in southern pine that is inside a 2×8 (11'-10"), with the 2×10 in hand if the beam has to move out.
| Southern pine | 12" o.c. | 16" o.c. | 24" o.c. |
|---|---|---|---|
| 2x6 | 9'-11" | 9'-0" | 7'-7" |
| 2x8 | 13'-1" | 11'-10" | 9'-8" |
| 2x10 | 16'-2" | 14'-0" | 11'-5" |
| 2x12 | 18'-0" | 16'-6" | 13'-6" |
Every other species group is in the full span chart.
Where the span stops governing
Three things quietly override the table. Composite decking often wants tighter centres than the joist can span — check the board maker's instruction, because a diagonal layout usually drops you to 12-inch centres. A hot tub or a planter is a point load the table never saw. And a joist that passes on span can still bounce; if the deck will hold a table of people, size up or tighten the spacing before you frame it.
The span also decides what has to catch it at the other end — see beam span and footing depth.
Sources
Next on the pile
Deck joist span chart
Every species and size at 12, 16 and 24 inch centres.
What size joist for a deck
Picking 2×6 through 2×12 from span, spacing and decking.
Deck joist spacing
What 12, 16 and 24 inch centres each cost you.
Deck beam span calculator
Beam size and post spacing for the span you just set.
Deck board calculator
Boards, linear feet and screws once the frame is sized.