What size post do I need for my deck?

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.

Can I use 4x4 posts for my deck?

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.

When do I need 8x8 posts?

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.

How does post height affect size?

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.

Do I need an engineer for tall posts?

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.

What's the footing size for each post?

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.

Can I use engineered steel posts?

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.

Should all my posts be the same size?

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.

Do post anchors affect post sizing?

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.

Material · IRC R507.4 post sizing

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.

IRC R507.4Buckling-aware4 post options

4·Post sizes

Auto·Footing dia

8-14ft·Height table

SF check·Always

Inputs

Beam spanft post-to-post

Joist spanft (perp.)

Load (psf)40+10 IRC

psf

Snow (psf)region

psf

Post heightfooting top → beam

Post · 4×4 PT pine

4x4SF 1.41 · 3,120 lb load

Tributary area

12/2 × 8

Total load

per post

Allowable

@ 8ft height

Footing dia

sonotube Ø

IRC R507.4 sizing

4×4 PT pine — SF 1.41

IRC R507.4

Total load 3,120 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.

How to use

How to use the deck calculator in 3 steps.

Enter beam span + joist span

Beam span = post-to-post horizontal distance. Joist span = perpendicular floor span. Tributary area = (joist span / 2) × beam span.

Enter load + snow

50 PSF default IRC. Snow PSF region-specific (use Snow Load Calculator). Effective load = live + dead + 50% snow.

Enter post height

From footing top to beam underside. Critical for column buckling — capacity drops 50%+ from 8ft to 14ft on same post size.

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 the result — 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

IRC 2021 R507.4 + AWC DCA-6 prescriptive post-load tables. Pricing 2026-Q1 retail.

Tributary area

tribArea = (joist_span / 2) × beam_span

Half the joist span × full beam span. For 12ft joist + 8ft beam: 6 × 8 = 48 sqft tributary per post.

Total load on post

load = tribArea × (live + dead + snow × 0.5)

48 sqft × 65 PSF effective = 3,120 lb. Compare to post capacity from IRC R507.4 table.

Post capacity (IRC R507.4)

allowable = table[post_size][height]

6×6 @ 8ft = 14,000 lb. 6×6 @ 14ft = 7,500 lb. Capacity drops with height (column buckling).

Footing diameter

footing scales with post load

Up to 4,000 lb → 12″ footing. 8,000 → 14″. 12,000 → 16″. Use Footing Diameter Calculator for full math.

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Deck calculator questions, answered.

Deck Post Spacing Calculator

IRC R507.5 beam-to-post spacing sizer. 12 beam profiles (double/triple 2×8/10/12, solid 4×6 to 6×10), 4 species, tributary + load. Post count + post size + Simpson connector spec. DCA-6 backed.

Footing Diameter Calculator

Footing diameter sizing — sonotube + Bigfoot Ø by post tributary load + soil bearing capacity. IRC R403.1.4 + R507.3.1 compliance. 8″-24″ Ø range × 5 soil tiers.

Deck Load Calculator

Reverse-check 'how much weight can my deck hold?' — joist + beam + post + footing vs live + dead + snow + point loads per IRC R301.5 + R507. Hot tub + outdoor kitchen + planter presets.