Deck Beam Span Calculator (IRC R507.5)
A deck beam's allowable span between posts depends on the beam size and the joist span it supports. Enter your deck and the calculator pulls the maximum post spacing from IRC Table R507.5 and tells you how many posts you need.
Member sizes read from the IRC R507.6 (joists) and R507.5 (beams) tables for No. 2 Southern Pineat 40 psf live + 10 psf dead. Footings from tributary load ÷ soil bearing (R507.3); ledger fasteners per R507.9. Confirm with your local building department.
| Decking — Pressure-treated pine192 sq ft · ~28 × 16′ boards | $575–$1,730 |
| Substructure (joists, beam, posts, hardware)192 sq ft framing | $1,150–$2,495 |
| Footings (concrete piers)4 posts | $140–$360 |
| Railing / guard40 lin ft | $600–$2,400 |
| Stairs4 treads | $120–$380 |
| Permit + fasteners + miscallowance | $75–$500 |
| Total estimate | $2,660–$7,865 |
Low = DIY material budget; high = contractor-installed (labor included). Regional prices vary — treat as a planning range, not a quote.
Full deck beam span table (IRC R507.5)
| Beam | 6′ | 8′ | 10′ | 12′ | 14′ | 16′ | 18′ |
|---|---|---|---|---|---|---|---|
| 2×2x6 | 6'11" | 5'11" | 5'4" | 4'10" | 4'6" | 4'3" | 4' |
| 2×2x8 | 8'9" | 7'7" | 6'9" | 6'2" | 5'9" | 5'4" | 5' |
| 2×2x10 | 10'4" | 9' | 8' | 7'4" | 6'9" | 6'4" | 6' |
| 2×2x12 | 12'2" | 10'7" | 9'5" | 8'7" | 8' | 7'6" | 7' |
| 3×2x8 | 10'10" | 9'6" | 8'6" | 7'9" | 7'2" | 6'8" | 6'4" |
| 3×2x10 | 13' | 11'3" | 10' | 9'2" | 8'6" | 7'11" | 7'6" |
| 3×2x12 | 15'3" | 13'3" | 11'10" | 10'9" | 10' | 9'4" | 8'10" |
| Beam | 6′ | 8′ | 10′ | 12′ | 14′ | 16′ | 18′ |
|---|---|---|---|---|---|---|---|
| 2×2x6 | 5'5" | 4'8" | 4'2" | 3'10" | 3'6" | 3'1" | 2'9" |
| 2×2x8 | 6'10" | 5'11" | 5'4" | 4'10" | 4'6" | 4'1" | 3'8" |
| 2×2x10 | 8'4" | 7'3" | 6'6" | 5'11" | 5'6" | 5'1" | 4'8" |
| 2×2x12 | 9'8" | 8'5" | 7'6" | 6'10" | 6'4" | 5'11" | 5'7" |
| 3×2x8 | 9'8" | 8'6" | 7'7" | 6'11" | 6'5" | 6' | 5'8" |
| 3×2x10 | 12' | 10'5" | 9'4" | 8'6" | 7'10" | 7'4" | 6'11" |
| 3×2x12 | 13'11" | 12'1" | 10'9" | 9'10" | 9'1" | 8'6" | 8'1" |
Deck Beam Span — what to know
- •Beam span is driven by the supported joist span: the longer the joists, the more load on the beam, the shorter it can span.
- •A 3-ply beam spans noticeably further than a 2-ply of the same depth — often the cheapest way to drop a post.
- •Built-up beams must be fastened per IRC R507.5.2; a dropped beam needs post-to-beam connectors rated for uplift.
Want a permit-ready deck plan?
This estimate is built for planning. For your permit packet, DeckCalc HQ Pro turns these numbers into a stamped framing plan, footing schedule and material list as a printable PDF.
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