How to use the joist span calculator
- Pick the wood species group printed on the lumber stamp. Most pressure-treated lumber in the US is southern pine; in the West and Canada it's often hem-fir or SPF.
- Choose the joist size and spacing you plan to use.
- Enter your span, the distance between the supports the joist rests on. The calculator checks it against the code maximum, finds the smallest joist that works, and the widest spacing you could use.
- Switch to "With cantilever" if the joists extend past the beam. The allowed back span is shorter for small joists.
Where the numbers come from
Spans come from the deck joist span table in the International Residential Code (Table R507.6 in the 2018 and 2021 IRC). They assume No. 2 grade lumber with a wet-service factor, a 40 psf live load, 10 psf dead load and a deflection limit of L/360 on the main span (L/180 on the cantilever, with a 220 lb point load at its end). Cantilevers are limited to one quarter of the actual back span.
These values cover decks in areas with a ground snow load up to 40 psf. If you're somewhere snowier, or your deck will carry a hot tub, planters or other heavy loads, use your local code's higher-load tables or an engineer.
Deck joist span chart (no cantilever)
| Species | Size | 12″ o.c. | 16″ o.c. | 24″ o.c. |
|---|---|---|---|---|
| Southern pine | 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″ | |
| Douglas fir, hem-fir, SPF | 2x6 | 9′ 6″ | 8′ 8″ | 7′ 2″ |
| 2x8 | 12′ 6″ | 11′ 1″ | 9′ 1″ | |
| 2x10 | 15′ 8″ | 13′ 7″ | 11′ 1″ | |
| 2x12 | 18′ 0″ | 15′ 9″ | 12′ 10″ | |
| Redwood, cedar, ponderosa, red pine | 2x6 | 8′ 10″ | 8′ 0″ | 7′ 0″ |
| 2x8 | 11′ 8″ | 10′ 7″ | 8′ 8″ | |
| 2x10 | 14′ 11″ | 13′ 0″ | 10′ 7″ | |
| 2x12 | 17′ 5″ | 15′ 1″ | 12′ 4″ |
Joist span chart with cantilever
Maximum back span when the joists also cantilever past the beam.
| Species | Size | 12″ o.c. | 16″ o.c. | 24″ o.c. |
|---|---|---|---|---|
| Southern pine | 2x6 | 6′ 8″ | 6′ 8″ | 6′ 8″ |
| 2x8 | 10′ 1″ | 10′ 1″ | 9′ 8″ | |
| 2x10 | 14′ 6″ | 14′ 0″ | 11′ 5″ | |
| 2x12 | 18′ 0″ | 16′ 6″ | 13′ 6″ | |
| Douglas fir, hem-fir, SPF | 2x6 | 6′ 3″ | 6′ 3″ | 6′ 3″ |
| 2x8 | 9′ 5″ | 9′ 5″ | 9′ 1″ | |
| 2x10 | 13′ 7″ | 13′ 7″ | 11′ 1″ | |
| 2x12 | 18′ 0″ | 15′ 9″ | 12′ 10″ | |
| Redwood, cedar, ponderosa, red pine | 2x6 | 5′ 7″ | 5′ 7″ | 5′ 7″ |
| 2x8 | 8′ 6″ | 8′ 6″ | 8′ 6″ | |
| 2x10 | 12′ 3″ | 12′ 3″ | 10′ 7″ | |
| 2x12 | 16′ 5″ | 15′ 1″ | 12′ 4″ |
Worked examples
Southern pine joists at 16″ on centre, spanning 12 ft from the ledger to the beam. A 2×8 tops out at 11′ 10″, so it fails. A 2×10 (14′ 0″ max) works.
Hem-fir 2×8s spanning 9 ft. At 16″ centres the limit is 11′ 1″, and at 24″ it's 9′ 1″, so you could space them at 24″, as long as the decking is rated for 24″ spans (2× wood is; 5/4 boards and most composites aren't).
Frequently asked questions
How far can a 2×8 deck joist span?
At 16″ on centre: 11′ 10″ in southern pine, 11′ 1″ in Douglas fir, hem-fir or SPF, and 10′ 7″ in redwood or cedar. At 12″ on centre the spans increase to 13′ 1″, 12′ 6″ and 11′ 8″.
How far can a 2×10 span?
At 16″ on centre: 14′ 0″ in southern pine, 13′ 7″ in Douglas fir, hem-fir or SPF, and 13′ 0″ in redwood or cedar.
Is a joist span measured centre to centre?
No. Code span tables use the clear span: the distance between the faces of the supports. For a ledger-attached deck, that's from the face of the ledger to the near face of the beam.
How far can deck joists cantilever?
Under the IRC, up to one quarter of the actual back span, and the back span must not exceed the cantilever table above. A 12 ft back span allows up to a 3 ft cantilever.