What is the maximum post spacing for a deck?

12 feet absolute per IRC R507.4.2 (prescriptive). Math may allow more (e.g., triple 2×12 SYP at 6-ft tributary = 8.5 ft baseline), but anything >12 ft requires engineered design (PE stamp). Most residential decks fall in the 6-9 ft range depending on beam size + tributary. A triple 2×10 SYP @ 7-ft tributary 40 psf = ~7 ft max spacing. Reduce tributary by adding intermediate beam to gain wider spacing.

How do I calculate post spacing for a deck beam?

Three inputs drive the math: beam size (depth + width), species (strength), tributary width (the joist span the beam supports). Formula: baseSpacing × speciesMultiplier × (6 / tributaryFt)^0.55. Base spacings from AWC DCA-6: triple 2×10 SYP = 7.5′ base (at 6′ tributary). For 8′ tributary: 7.5 × (6/8)^0.55 = 6.4′. For Hem-Fir: 6.4 × 0.92 = 5.9′. Round down. Apply IRC R507.4.2 absolute cap of 12′.

What size post do I need for a deck?

Depends on deck height per IRC R507.5.1. 4×4 PT — acceptable for low decks ≤4 ft height. 6×6 PT — residential standard for 4-10 ft height (most decks). 8×8 PT — required for ≥10 ft height OR per-post load >8,000 lb. Plus: chemical treatment must be UC4B (heavy-duty, ground-contact) for any in-ground post. Coastal/marine: stainless 316 connectors mandatory. Some jurisdictions require 6×6 minimum regardless of height — check local code.

Can I space deck posts 10 feet apart?

Yes if your beam supports it within IRC R507.4.2 12-ft cap. Required: large beam + low tributary + best species. Examples that work at 10 ft spacing: 6×10 SYP at 6 ft tributary (math allows it); triple 2×12 SYP at 4 ft tributary (small wide deck). Most don't — typical 16×20 deck with 16′ tributary forces ~5-6 ft spacing on standard beams. To go wide: (a) upgrade to LVL or solid 6×10/6×12 beam, (b) add an intermediate beam to halve tributary, (c) get engineered design.

Do I need a beam cap connector for deck posts?

Yes — IRC R507.5 mandates positive connection between post and beam. Simpson PB44/PB66/PB88 post base at footing + matching beam cap on top. Beam caps: LCE strap-cap or BCS for built-up beams (double/triple 2×); AC adjustable cap or BC beam cap for solid beams (4×6, 6×6, etc). Toe-nailing alone is NOT code-compliant. ASTM A153 HDG ZMAX for modern MCA-treated PT; SS 316 for coastal/marine or ACQ-treated lumber. $25-85 per connection — non-negotiable.

How does joist tributary width affect post spacing?

Inverse relationship. Wider tributary = shorter post spacing. Why: a beam supporting wider joist spans carries more total load per foot of beam length. To carry more load, you either: (a) upsize the beam (more depth/width), or (b) shorten the post-to-post span (more posts). Tributary scaling exponent ~0.55 in DCA-6 tables — doubling tributary (6′ → 12′) drops max spacing by 32%. For wide decks (>16′ width), add a center beam between ledger and outer beam to halve tributary.

What's the per-post load on a deck footing?

Per-post load = tributary × spacing × (live load + dead load). Example: 8-ft tributary, 7-ft spacing, 50 psf total = 2,800 lb per post. Multiply by 1.5-2.5× safety factor for footing sizing. 12″ Ø sonotube footing (0.78 sq ft × 1,500 psf clay) = 1,200 lb capacity — undersized for this case. Need 16-18″ Ø footing. Run the Footing Depth Calculator with your per-post load to verify diameter + soil bearing. Hot tub decks: load doubles to ~6,000 lb per post — usually requires 24″ Ø footing or helical pile.

Can a single beam support a whole deck without intermediate posts?

Only on small decks. Single-span beam (just two end posts) requires the entire deck length to be within max post spacing. Examples: 8-ft deck with triple 2×10 at 6-ft tributary works at 7.5 ft max → 8 ft is over, needs 1 intermediate post. 10-ft deck at same spec needs 1 intermediate. For wide decks (>10 ft) you'll almost always have 2-3+ posts. For freestanding decks where the beam IS the perimeter (no ledger), the math gets more complex — often requires engineered design.

What's the difference between double, triple, and solid beams?

Built-up beams: 2-3 dimensional 2× lumber laminated face-to-face. Double 2×10 = 3″ thick × 9.25″ deep. Triple 2×10 = 4.5″ thick × 9.25″ deep — ~50% stronger than double of same depth. Glue + nail at 12″ o.c. minimum per AWC. Solid beams: single piece of larger nominal lumber (4×8 = 3.5″ × 7.25″; 6×8 = 5.5″ × 7.25″). Pros: cleaner aesthetic, fewer fastener concerns. Cons: more checking + heart-checks over time, harder to source (special-order for 6×10+), heavier to install. For most residential builds, built-up 2×10/2×12 wins on availability + cost.

When do I need an engineer for deck post spacing?

Required by IRC R507 + ACI 318 when: (a) post spacing >12 ft prescriptive cap, (b) tributary width >12 ft (large or non-standard decks), (c) live load >60 psf (commercial/heavy gathering), (d) deck height >10 ft (lateral wind/seismic loads dominate), (e) cantilevered post configurations (post off-center on beam), (f) hot tub or other concentrated loads >40 psf over a 12-sq-ft area. Engineer cost $400-1,500 typical for residential deck. Always cheaper than retrofitting an under-sized deck — get the engineer up front.

IRC R507.5 · 12 beam profiles · 4 species · DCA-6

Deck Post Spacing Calculator

Beam-to-post sizer per IRC 2021 R507.5 + AWC DCA-6. Pick beam (12 profiles from double 2×8 to solid 6×10), species (Southern Pine baseline, Douglas Fir-Larch +5%, Hem-Fir −8%, SPF −12%), joist tributary width, live load (40 / 60 psf), and deck length — get max allowable post spacing, post count, post-size recommendation (4×4 / 6×6 / 8×8 by deck height), Simpson connector spec (PB44/PB66/PB88 + matching beam cap), and per-post tributary load for footing sizing. IRC R507.4.2 absolute 12 ft cap applied. Pairs with Beam Span Calculator (beam sizing) and Footing Depth Calculator (per-post load → footing diameter).

12 beam profiles4 species40 + 60 psf loadsPost size 4×4 / 6×6 / 8×8Simpson connectorsIRC R507.5 + DCA-6Free forever

5-12′·Max spacing range

12 beams·Built-up + solid options

PB44/66/88·Simpson connector spec

12′ cap·IRC R507.4.2 absolute

Inputs

Beamsize + profile

Conditions

Speciesbending strength

Live loadpsf — IRC R301.5

Tributary widthjoist span / 2 + cant.

Deck lengthparallel to beam

Deck heightpost size driver

Desired spacing0 = use max

Max spacing6.89′

Posts at max4

Results

Triple 2×10 Southern Pine @ 7′ tributary · 40 psf · 16′ deck

Code-compliant spacing

Max 6.89′ between posts · 4 posts for 16′ deck

Triple 2×10 Southern Pine delivers 6.89′ at 7′ tributary. Use 4x4 posts with PB44 + LCE4.

Max spacing

math-derived

Post count

for 16′ deck

Post size

@ 36″ height

Per-post load

cross-check footing

Spacing 6.89′ passes IRC R507.5 (max 6.89′)

IRC 2021 R507.5 · AWC DCA-6

Triple 2×10 Southern Pine @ 7′ tributary, 40 psf supports up to 6.89′ between posts. Final design uses 6.89′ — 4 posts for 16′ deck.

Within IRC R507.4.2 12′ prescriptive cap

IRC R507.4.2

IRC R507.4.2 caps prescriptive beam span at 12 ft. Math may allow more for this beam; capped here.

Post size: 4x4 (deck height 36″)

IRC R507.5.1

4×4 OK for low deck height (≤4 ft). 6×6 recommended if you're staying long-term — adds rigidity for minimal cost.

Per-post load: 2,508 lb

IRC R507.3 · ACI 318

Standard 12″ Ø footing on average soil (~2,400 lb capacity) typically sufficient. Verify with Footing Depth Calculator + actual soil bearing test.

Connector: PB44 + LCE4

ASTM A153 HDG · Simpson Strong-Tie spec

PB44 Post Base + LCE4 L-Bracket Cap (2-piece system, $25-35)

Connector spec

PB44 + LCE4

PB44 Post Base + LCE4 L-Bracket Cap (2-piece system, $25-35)

Post base bolts to footing concrete via 1/2″ × 7″ anchor bolt (cast into wet concrete). Beam cap straps + nails the beam down to the post top. ASTM A153 HDG ZMAX for modern MCA-treated PT; stainless 316 required for ACQ-treated lumber or coastal/marine exposure. Always verify the bolt diameter matches the embedded anchor — mismatched = code violation.

Math breakdown

Base spacing (Triple 2×10 SYP @ 6′ tributary, 40 psf): 7.5′
× Southern Pine multiplier (1): 7.50′
× 40 psf load (1.00×): 7.50′
× tributary factor (6/7)^0.55: 0.919
Max spacing (capped at 12′): 6.89′
Post count = ⌈16/6.89⌉ + 1: 4 posts

How to use

Three steps. Permit-ready output.

Pick beam size

12 beam options from double 2×8 (light) to solid 6×10 (heavy timber). Most residential decks use triple 2×10 SYP (workhorse). Double 2×8 is light-duty mid-tier. Solid 4×6/4×8 common in PNW custom builds. 6×6+ is heritage/timber-frame aesthetic. The base max spacing column shows the SYP @ 6-ft tributary baseline before species and tributary adjustments.

Set species + tributary + load

Species: Southern Pine 1.00× (baseline), Douglas Fir-Larch 1.05× (PNW/west best), Hem-Fir 0.92× (8% less span), SPF 0.88× (12% less span — Northeast/Midwest cheap option). Tributary width = the joist length each beam supports — typically half the deck width plus any cantilever. For a 16′ wide deck with the beam at the edge: tributary = 8′. For center beam: 4-6′. Live load: 40 psf (residential default per R301.5) or 60 psf (hot tub/kitchen zones — 18% spacing derate).

Enter deck length + desired spacing

Deck length parallel to the beam drives the post count. Desired spacing (optional, 0 = use max) — useful for matching footing locations to existing features (deck stair landing, sliding door, etc). If desired > max, the calc warns + offers fix options (upsize beam, reduce tributary, reduce spacing).

Set deck height for post sizing

Deck height drives post-size recommendation: 4×4 OK ≤48″ (4 ft) per IRC R507.5.1 commentary, 6×6 standard for 4-10 ft height, 8×8 for ≥10 ft height + engineered footing design. The connector spec (Simpson PB44/PB66/PB88 + beam cap) auto-matches.

Read max spacing + post count + per-post load

Headline = max spacing in feet + post count for your deck length. Per-post load (lb) is your tributary × spacing × (LL + DL ≈ 50 psf) — cross-check with Footing Depth Calculator. 12″ Ø sonotube on 1,500-psf clay = ~1,200 lb capacity; need 16-18″ Ø footing for high-load configs.

How we calculate

The math, fully transparent.

The Post Spacing Calculator is the IRC R507.5 + AWC DCA-6 beam-to-post sizer — set beam size (12 options: double/triple 2×8/2×10/2×12, plus solid 4×6, 4×8, 4×10, 6×6, 6×8, 6×10), species (Southern Pine baseline, Douglas Fir-Larch +5%, Hem-Fir −8%, SPF −12%), joist tributary width, and live load (40 psf default / 60 psf hot tub) — get max allowable post spacing, post count for your deck length, post-size recommendation (4×4 / 6×6 / 8×8 by deck height), Simpson connector spec (PB44/PB66/PB88 + matching cap), and per-post tributary load with footing-size implications. IRC R507.4.2 absolute cap of 12 ft prescriptive applied. Pairs with Beam Span Calculator (beam sizing) + Footing Depth Calculator (sizing footings for per-post load).

IRC references

IRC 2021 R507.5 — Deck beam-to-post connection + spacing
IRC 2021 R507.4.2 — Maximum beam span (12 ft prescriptive; >12 ft requires engineered design)
IRC 2021 R507.5.1 — Post size requirements (4×4 ≤4 ft height; 6×6 above)
IRC 2021 R301.4-R301.5 — Live load (40 psf residential, 60 psf for occupancy >loaded areas)
AWC DCA-6 — Prescriptive Residential Wood Deck Construction Guide (beam span + post tributary tables)
ASTM A153 — Hot-dipped galvanized fastener spec for post bases + beam caps

AWC DCA-6 Prescriptive Residential Wood Deck Construction Guide, Southern Pine #2 baseline beam tables at 6-ft tributary, 40 psf live load. Species multipliers: SYP 1.00, Douglas Fir-Larch 1.05, Hem-Fir 0.92, SPF 0.88. Tributary scaling: max spacing × (6/tributaryFt)^0.55 (matches DCA-6 table progression). Live load derate: 60 psf reduces max spacing by 18%. Hard cap 12 ft per IRC R507.4.2 prescriptive; >12 ft requires PE-stamped engineered design. Post size: 4×4 ≤4 ft height, 6×6 4-10 ft, 8×8 >10 ft (R507.5.1 commentary). Simpson connector spec: PB44 post base + LCE/AC cap for 4×4; PB66 + BCS/BC for 6×6; PB88 + BC80 for 8×8 (ASTM A153 HDG ZMAX standard, SS 316 for coastal).

Max post spacing

maxSpacing = baseSpacing × speciesMult × loadMult × (6 / tributaryFt)^0.55

AWC DCA-6 beam tables baseline at 6-ft tributary, SYP, 40 psf. Scale by species multiplier (SYP 1.00 / DF-L 1.05 / HF 0.92 / SPF 0.88), live-load derate (60 psf = 0.82× spacing), and tributary scaling exponent 0.55 (matches DCA-6 table progression). Hard cap at 12 ft per IRC R507.4.2 prescriptive limit.

Post count

posts = ceil(deckLengthFt / spacing) + 1

End posts at both deck corners + interior posts between. A 16-ft deck at 7-ft spacing = ceil(16/7) + 1 = 3 + 1 = 4 posts (corners + 2 intermediate). For 8-ft spacing = 3 posts (corners + 1 middle).

Per-post load

perPostLb = tributaryFt × spacingFt × (LL + DL psf)

Each post carries the load of (its half-span on each side × its tributary depth) at total (LL + DL) psf. Example: 8-ft tributary, 7-ft spacing, 50 psf total = 2,800 lb per post. This is the load the footing + post must transmit to soil. Cross-check footing diameter using soil bearing × area ≥ this load × safety factor 1.5-2.5.

Post size recommendation

≤48″ → 4×4; ≤120″ → 6×6; >120″ → 8×8

IRC R507.5.1 commentary: 4×4 PT acceptable for low decks (height ≤4 ft). 6×6 is the residential standard for typical decks (4-10 ft). 8×8 required for tall decks (>10 ft) or where per-post load >8,000 lb. Lateral-load resistance + buckling capacity drive the upgrade thresholds.

Simpson connector matching

Post base + beam cap matched to post size + beam profile

Post base: PB44 (4×4), PB66 (6×6), PB88 (8×8) — bolts to footing concrete anchor. Beam cap matches beam profile: built-up beams use LCE strap cap or BCS beam-cap straps; solid beams use AC adjustable cap or BC beam cap. ASTM A153 hot-dipped galvanized (HDG ZMAX) for modern MCA PT; stainless for ACQ or coastal exposure.

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Deck Beam Span Calculator

Maximum beam span (post-to-post) by size, ply, species, and tributary load — straight from IRC R507.5 / DCA-6.

Joist Span Calculator

Maximum joist span for any size, species, and spacing — directly from IRC 2021 R507.6 tables.

Footing Depth Calculator

Per-footing IRC R507.3 sizer: depth from frost + structural, diameter from soil bearing, rebar spec, concrete order. 5 soils × 4 footing types.

Deck Footing Calculator

Footing diameter, depth, concrete volume, and bag count — IRC R507.3 sized + frost depth lookup for all 50 states.