Deck joist spans depend on three choices: the lumber size, how far apart the joists sit, and what species the lumber is. The chart reproduces AWC DCA 6 Table 2, the prescriptive deck guide most building departments accept, for a 40 psf live load and 10 psf dead load on No. 2 grade lumber in wet service.
Reading the table
The allowable span LJ is measured from the face of the ledger (or support) to the face of the beam, not including any overhang. Your joist span must be at or under the value for your size, spacing and species. A Southern Pine 2x8 at 16" spans 11'-10"; the same joist in Douglas fir-larch, hem-fir or SPF spans 11'-1", and in redwood or western cedar 10'-7".
DCA 6 groups Douglas fir-larch, hem-fir and spruce-pine-fir together (incised, as treated lumber usually is), and redwood, western cedars, ponderosa pine and red pine together. 2x12 joists at 12" are capped at 18'-0" so the footing tables stay valid.
Overhang: the lesser of the table and L/4
Joists may cantilever past the beam by the allowable overhang LO or one quarter of the actual main span, whichever is less. A 10 ft span can overhang 2'-6" at most even if the table allows more; a long overhang is also limited by the bounce of a 220 lb point load at its tip, which the table already includes.
overhang ≤ lesser of LO and L ÷ 4
Spacing and decking
Closer joists span further and stiffen the deck surface. Wood 5/4 decking is commonly run on 16" centres; many composite boards need 16" straight or 12" when laid diagonally, so check the decking first and let it set the spacing.
Ledger and hangers
DCA 6 requires a 2x8 minimum ledger, and joists or rims that carry guard posts must be 2x8 or larger. Joist hangers must be at least 60% of the ledger or beam depth and rated for 400 lb (2x6), 500 lb (2x8), 600 lb (2x10) or 700 lb (2x12). Use corrosion-resistant hangers and the fasteners the maker lists for treated lumber.
When these spans do not apply
The table assumes a 40 psf live load. Hot tubs, planters, heavy snow regions, roof loads on the deck and anything over 18 ft are outside it and need engineering or your local span tables.