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Deck Foundation & Post Engine

Deck Post Calculator

Estimate how many vertical support posts, commercial stock pieces, and linear lumber you need for your deck substructure based on dimensions, post spacing, deck height, and framing layout.

Deck Post Specifications

Enter your deck dimensions, post spacing along the beam, height, and stock lumber length.

Used for local currency and verified footing concrete datasets.

Deck Dimensions

ft
ft

Framing Layout & Post Spacing

Ledger-attached decks use house ledger for rear support, requiring posts only on front beam.

ft

Standard residential post spacing is typically 6 to 8 feet (1.8 to 2.4 m).

Post Height & Lumber Stock Settings

ft

Height from concrete footing top to underside of deck beam.

ft

Available commercial 4x4 or 6x6 stock post lengths: 8', 10', 12' (2.4–3.6 m).

%

Leave blank for 0% waste. 10% is standard to round up commercial lumber stock pieces.

How to Calculate Deck Posts: Formulas & Spacing Rules

Deck support posts transfer the gravity loads of the deck, furniture, and occupants from the supporting beams down into concrete footings and the soil. Determining post quantities requires calculating the number of spans along your beam lines.

Authoritative Deck Post Calculation Formulas:

1. Bays Along Beam Line = ⌈ Beam Length ÷ Post Spacing ⌉

2. Posts Per Beam Line = Bays Along Beam Line + 1

3. Ledger-Attached Deck Posts = Posts Per Beam Line (1 Beam Line)

4. Freestanding Deck Posts = 2 × Posts Per Beam Line (2 Beam Lines)

5. Perimeter Layout Posts = (2 × Length Posts) + (2 × Width Posts) - 4 Corners

6. Cuts Yielded Per Stock Piece = ⌊ Stock Lumber Length ÷ Deck Cut Height ⌋

7. Commercial Stock Posts to Purchase = ⌈ (Base Posts ÷ Cuts Per Piece) × (1 + Waste % ÷ 100) ⌉

Original Worked Example (Verified Arithmetic)

Example: 20 ft Length × 12 ft Width Ledger-Attached Deck

  • Inputs: 20 ft Length, 12 ft Width, 8 ft Post Spacing, 4 ft Deck Height, 8 ft Stock Lumber, 10% Waste, Ledger-Attached layout.
  • Beam Length: 20 feet.
  • Bays along Beam: ⌈20 ft ÷ 8 ft⌉ = 3 bays.
  • Base Support Posts: 3 bays + 1 = 4 support posts along outer drop beam.
  • Cuts per Stock Piece: ⌊8 ft stock ÷ 4 ft height⌋ = 2 posts per 8 ft board.
  • Base Stock Boards: ⌈4 posts ÷ 2⌉ = 2 stock boards.
  • With 10% Waste: ⌈2 × 1.10⌉ = 3 stock boards to purchase (+1 extra board).
  • Total Linear Material: 4 posts × 4 ft = 16.0 linear ft (with 10% waste = 17.6 linear ft).
  • Concrete Footing Benchmark: 4 footing holes at \$15–\$38 per post = \$60.00 to \$152.00.

4×4 vs 6×6 Deck Post Sizing Guidelines

Post Dimension Maximum Height (IRC Code) Recommended Beam Connection Code Suitability
4×4 Post (3.5" × 3.5") Up to 6–8 feet (check local code) Notched post cap bracket Permitted on low, single-level ground decks; prohibited in many jurisdictions for multi-story decks.
6×6 Post (5.5" × 5.5") Up to 14 feet Direct bearing notched shoulder or Simpson post cap Standard requirement for elevated decks, multi-ply built-up beams, and modern residential building permits.

Frequently Asked Questions

How many deck posts do I need for a 20×12 deck?

For a standard 20ft × 12ft ledger-attached deck with an 8-foot beam span, you need 4 support posts along the outer drop beam (ceil(20 ÷ 8) + 1 = 4 posts). For a freestanding deck without a house ledger, you need 2 beam lines for a total of 8 support posts.

What is the standard spacing between deck posts?

Standard residential post spacing is typically 6 to 8 feet along the supporting beam line. Actual allowable post spacing depends on your beam size (e.g., double 2x8 vs triple 2x10), joist span length, and local snow/live load building codes.

Should I use 4x4 or 6x6 deck posts?

Most modern residential building codes (such as the IRC) require 6x6 posts for all decks elevated higher than 6 feet above grade, or when supporting multi-ply drop beams. 4x4 posts are generally permitted only on low-profile, ground-level decks under 6 feet in height.

How does deck height affect the number of post lumber boards to purchase?

If your deck is low to the ground (e.g., 3 to 4 feet tall), a single 8-foot commercial stock post can be cut in half to yield 2 finished support posts. If your deck is tall (e.g., 8 to 10 feet), you will need one full commercial post piece for each post location.

Does this calculator determine concrete footing depth or diameter?

No. This calculator is strictly a material quantity estimator. Concrete footing diameter (typically 12" to 18") and depth depend on your local frost line depth (ranging from 12" in warm climates to 48"+ in northern regions) and local soil bearing capacity.

What is the difference between a ledger-attached deck and a freestanding deck layout?

A ledger-attached deck is bolted directly to the house rim joist with an approved ledger board, requiring posts only along the outer front beam. A freestanding (floating) deck is not anchored to the house and requires supporting posts and beams on both the front edge and the rear house-side edge.

Related Deck & Framing Calculators

Deck Material Calculator → Deck Board Calculator → Deck Joist Calculator → Deck Cost Calculator → Wood Deck Cost Calculator → Composite Deck Cost Calculator →

Structural & Engineering Disclaimer: This deck post calculator provides a planning-level material quantity estimate. It does not calculate soil bearing capacity, footing depth or diameter, frost-heave resistance, beam cantilever overhangs, or vertical post load-bearing limits. Always verify your foundation and post specifications with your local building code official or a licensed structural engineer before digging footings or pouring concrete.