Deck Post Size Calculator
Size deck posts given tributary load + post height. 4×4 / 6×6 / 8×8 / engineered steel recommendation per IRC R507.4 prescriptive table. Column-buckling-aware: capacity drops with height (6×6 = 14,000 lb at 8ft → 7,500 lb at 14ft). Auto-suggests footing diameter to match. Different from Post Spacing — that one assumes 6×6 and asks spacing; this one asks WHAT POST.
Inputs
IRC R507.4 sizing
4×4 PT pine — SF 1.47
IRC R507.4Total load 3,000 lb. Post capacity 4,400 lb at 8ft height. 12″ Ø footing recommended.
Cost reference: 4×4 $4.20/LF · 6×6 $9.80 · 8×8 $18.50 · Engineered steel $35.00 (2026-Q1). Pair with Footing Diameter + Post Spacing + Soil Bearing Calculators for full base sizing.
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How to use
How to use the post size calculator in 4 steps.
- 1
Enter beam span + joist span
Beam span = post-to-post horizontal distance. Joist span = perpendicular floor span. Tributary area = (joist span / 2) × beam span.
- 2
Enter load + snow
50 PSF default IRC. Snow PSF region-specific (use Snow Load Calculator). Effective load = live + dead + 50% snow.
- 3
Enter post height
From footing top to beam underside. Critical for column buckling — capacity drops 50%+ from 8ft to 14ft on same post size.
- 4
Read recommended post + footing
Calculator picks smallest IRC-compliant post + sizes the footing accordingly. Posts over 10 ft trigger 'needs engineering' flag.
How we calculate
How DeckMath calculates this — IRC 2021 sources.
The Post Size Calculator sizes deck posts given tributary load + post height (different from Post Spacing — which assumes 6×6 + asks spacing). Returns recommended post (4×4 / 6×6 / 8×8 / engineered steel) + safety factor + required footing diameter. Per IRC R507.4 prescriptive table — column buckling matters: a 6×6 carries 14,000 lb at 8 ft post height but only 7,500 lb at 14 ft. Auto-flags posts over 10 ft as needs-engineering territory.
IRC references
- IRC R507.4 — Deck post sizing
- AWC DCA-6 — Prescriptive Deck Guide
Verify against the published source: 2021 International Residential Code (ICC).
IRC 2021 R507.4 + AWC DCA-6 prescriptive post-load tables. Pricing 2026-Q1 retail.
Half the joist span × full beam span. For 12ft joist + 8ft beam: 6 × 8 = 48 sqft tributary per post.
48 sqft × 65 PSF effective = 3,120 lb. Compare to post capacity from IRC R507.4 table.
6×6 @ 8ft = 14,000 lb. 6×6 @ 14ft = 7,500 lb. Capacity drops with height (column buckling).
Up to 4,000 lb → 12″ footing. 8,000 → 14″. 12,000 → 16″. Use Footing Diameter Calculator for full math.
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People also ask
Post size questions, answered.
Depends on tributary load + post height. For typical residential deck (12ft joist span × 8ft beam span × 50 PSF + snow): tributary load ~3,100 lb. At 8 ft post height, 6×6 PT is the standard recommendation (capacity 14,000 lb = 4.5× safety factor). 4×4 is undersized for most decks. 8×8 only for very heavy loads or tall posts.
Only for very small low decks. 4×4 PT capacity drops fast with height: 4,400 lb at 8 ft → 1,300 lb at 14 ft. Practical use cases: ground-level decks (post height ≤ 3 ft), bump-outs with low load. For ANY house-attached deck above 30″ height, 6×6 is the safer minimum. The cost difference ($4 vs $10 per LF) is trivial compared to the safety margin.
Heavy loads or tall posts. 8×8 capacity: 28,000 lb at 8 ft → 18,000 lb at 14 ft. Use cases: (1) hot tub + outdoor kitchen above the post, (2) post height > 12 ft, (3) heavy snow zones + large tributary, (4) commercial decks. ~$18.50/LF vs $9.80 for 6×6 — premium pays off in safety + future-load capacity.
Significantly — column buckling. Same 6×6 PT: 14,000 lb at 8 ft → 7,500 lb at 14 ft (46% capacity loss). The slenderness ratio L/d gets worse with height. For posts over 10 ft, the calc flags 'needs engineering' because buckling becomes more complex than the prescriptive table allows.
Yes for posts > 14 ft (calculator hard-flag) or > 10 ft with heavy loads. AWC DCA-6 prescriptive tables go to 14 ft max. Beyond that: bracing requirements (lateral X-braces, K-braces) + connection details get complex. SE letter $400-800. Engineered steel posts ($35/LF) are another option for very tall residential builds.
Scales with tributary load. Up to 4,000 lb → 12″ Ø sonotube. 8,000 lb → 14″. 12,000 → 16″. 18,000 → 18″. The calculator suggests the footing diameter automatically. Use the Footing Diameter Calculator for soil-bearing-specific sizing (different soil = different capacity). Bigfoot bell-base footings give more bearing area at smaller tube diameter.
Yes — 50,000+ lb capacity at any reasonable height. Common brands: Postmaster, Sturdi-Built. Cost ~$35/LF vs $9.80 for 6×6 PT. Use cases: (1) very tall posts (> 14 ft hillside decks), (2) modern/architectural designs (slim steel posts look intentional), (3) very heavy loads (4-person hot tub + outdoor kitchen). Adds aesthetic + capacity at premium cost.
Yes for visual consistency + simplicity. Even if some posts carry lighter loads, using same size everywhere makes inspector approval easier + framing simpler. The calculator's recommendation should be applied to ALL posts on the deck. Don't mix 6×6 corner posts with 4×4 mid posts unless you have a stamped design with specific layout reasons.
Yes — anchor capacity must match post + load. Simpson ABU66Z (for 6×6) rated 5,000-7,000 lb (varies). ABU88Z (for 8×8) higher. If post tributary > anchor capacity, use upgraded anchor (CPT for higher capacity). The calculator's post recommendation pairs with standard Simpson ABU for that post size — verify anchor capacity matches your load if you exceed prescriptive limits.
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