Deck Joist Span Calculator
Enter the distance your joists need to cover. See whether your joist size and spacing pass the 2021 IRC deck table, and which alternatives would.
IRC maximum span
11′ 10"
- Your 11′ 6" span passes
4" to spare.
- Joists needed
- 13
- Max cantilever (¼ span)
- 2′ 10 1/2"
Sizes that work for 11′ 6": 2×8 @ 12", 2×8 @ 16", 2×10 @ 12", 2×10 @ 16", 2×12 @ 12", 2×12 @ 16"
Deck joist span chart (IRC Table R507.6)
Maximum joist span, No. 2 grade, 40 psf live + 10 psf dead load, wet service. Your current selection is highlighted.
| Species | Size | 12" o.c. | 16" o.c. | 24" o.c. |
|---|---|---|---|---|
| Southern pine | 2×6 | 9′ 11" | 9′ 0" | 7′ 7" |
| 2×8 | 13′ 1" | 11′ 10" | 9′ 8" | |
| 2×10 | 16′ 2" | 14′ 0" | 11′ 5" | |
| 2×12 | 18′ 0" | 16′ 6" | 13′ 6" | |
| Douglas fir-larch, hem-fir, spruce-pine-fir | 2×6 | 9′ 6" | 8′ 8" | 7′ 2" |
| 2×8 | 12′ 6" | 11′ 1" | 9′ 1" | |
| 2×10 | 15′ 8" | 13′ 7" | 11′ 1" | |
| 2×12 | 18′ 0" | 15′ 9" | 12′ 10" | |
| Redwood, western cedars, ponderosa pine, red pine | 2×6 | 8′ 10" | 8′ 0" | 6′ 10" |
| 2×8 | 11′ 8" | 10′ 7" | 8′ 8" | |
| 2×10 | 14′ 11" | 13′ 0" | 10′ 7" | |
| 2×12 | 17′ 5" | 15′ 1" | 12′ 4" |
Source: 2021 IRC Table R507.6. Incised pressure-treated lumber and heavy loads (hot tubs, snow over 40 psf) reduce allowable spans; ask your building department.
Why closer spacing lets a joist reach farther
Each joist carries a strip of deck as wide as the spacing. Halve the spacing and each joist carries half the load, so it can span farther before it bends too much. Pick a spacing to see the load strip and how far a 2×10 southern pine joist may reach.
2×10 southern pine at 16" o.c.: 14′ 0" maximum span. Stiffness, not strength, is usually what limits a deck joist: the IRC table caps deflection so the deck does not feel springy underfoot.
Deck post spacing
Posts hold up the beam, so post spacing is really a beam span. It depends on three things: the beam (how many plies, how deep), the species, and the joist span feeding the beam. Longer joists put more weight on the beam, so posts must come closer. In practice most ledger-attached decks land at 6 to 8 ft between posts with a 2-ply 2×10 or 2×12 beam, and a 3-ply beam stretches that further.
- Look up your beam in IRC Table R507.5 (or the AWC DCA 6 guide), reading the column for your joist span. Don't guess from another deck.
- Beam plies should break only over a post, and the beam can overhang the post by up to a quarter of its span.
- Use 6×6 posts under beams: R507.4 caps 4×4 posts at short heights, and most inspectors want 6×6 so the beam can be notched in and through-bolted.
Joist spans feed straight into the deck calculator, which counts posts, footings and concrete for the spacing you choose.
Ask the shop
Stuck on a detail? Ask a question about this job. Your current inputs and results are sent along so the answer fits your numbers. AI answers can be wrong, so double-check anything structural with your building department.