Introduction
These are the questions deck builders ask most about posts, spacing, and footings — grouped by topic so you can jump straight to what you need. For full derivations behind the short answers, see the Deck Post Calculator and its companion guides; each answer below links onward where a deeper explanation helps.
Planning and Counts
How many posts does my deck need?
Count support points under the beams: (beam length / post spacing) + 1 per beam, times the number of beams, plus guardrail posts around the exposed perimeter. A 12×16 ft (3.66×4.88 m) deck with two beams at 8-ft (2.44 m) spacing needs 6 structural posts plus railing posts.
Do I count guardrail posts separately?
Yes. Guardrail posts are spaced up to 6 ft (1.83 m) apart around the exposed edges and are additional lumber beyond the structural posts under the beams. Forgetting them is one of the most common estimating errors — see the common mistakes guide.
What is the formula for post spacing?
Spacing is not calculated from the deck — it is limited by beam span tables (IRC R507) for your beam size and species, typically 6-8 ft (1.83-2.44 m) for residential decks. You choose a spacing at or under the table maximum, then derive the post count from it.
How do I lay out post locations on the ground?
Establish the beam lines, square them with the 3-4-5 triangle method, mark post centers with stakes, and use batter boards with string lines so the layout survives digging. The pro tips guide covers the full sequence.
Sizing: 4×4 vs 6×6
Should I use 4×4 or 6×6 posts?
6×6 is the modern standard for elevated decks — greater buckling resistance and better connections. 4×4 remains acceptable for low, ground-level decks within code limits. The material guide compares lumber options in detail.
What are the actual dimensions of a 4×4 and 6×6?
A nominal 4×4 measures 3.5 x 3.5 inches (89 x 89 mm); a nominal 6×6 measures 5.5 x 5.5 inches (140 x 140 mm). Always use actual dimensions when sizing connectors and checking bearing on footings.
How tall can a deck post be?
Post height limits depend on post size, bracing, and local code — tall 4×4 posts are prone to buckling, which is why elevated decks call for 6×6 stock, often with diagonal bracing. Confirm maximum unbraced heights with your local building department.
Can I splice two short posts into one tall post?
No. Structural deck posts must be single continuous pieces from footing to beam. Buy the next standard lumber length up and trim to fit.
Footings and Concrete
How big should deck footings be?
Typically 12 inches (30.5 cm) in diameter or larger for 4×4 posts, and bigger for 6×6 posts, heavy loads, or soft soils. The footing must fully support the post base connector with bearing area to spare.
How deep must footings go?
Below your local frost line — this ranges from under a foot in warm climates to 4 ft (1.22 m) or more in northern regions. Your building department publishes the local number; it is a code requirement, not a recommendation.
How much concrete per footing?
Volume = pi x r^2 x h. A 12-in x 36-in (30.5 x 91 cm) footing holds about 2.36 ft^3 (0.0667 m^3) — roughly four 80-lb (36 kg) bags. Multiply by your footing count and add 5-10% waste.
Can a post sit directly in the concrete?
It is far better to use a standoff post base connector that holds the wood at least 1 inch (25 mm) above the concrete. Direct burial of end grain in concrete or soil invites rot. Set the anchor bolt while the concrete is wet.
Materials and Code
Pressure-treated or cedar for posts?
Pressure-treated pine rated for ground contact is the default structural choice — strong and economical. Cedar costs roughly double but resists rot naturally and looks better where posts are visible. Never use non-rated lumber for structural posts.
Should beams be notched into posts or bolted?
Through-bolting or engineered beam-to-post caps are preferred because they preserve the post’s full cross-section. Notching is restricted or prohibited in many jurisdictions — check local code before cutting.
Do I need a permit for deck posts and footings?
In most jurisdictions, yes. Expect a footing inspection before you pour, a framing inspection, and a final inspection. The code compliance guide outlines the IRC R507 basics and the permit process.
What size footing does a 6×6 post need?
Larger than a 4×4 footing — typically 16 to 18 inches (40.6 to 45.7 cm) in diameter for residential work, sized to the load and the soil’s bearing capacity, and always below the frost line. The post base connector must sit fully on the concrete with margin to spare on every side.
How much do deck posts and footings cost?
As a planning range: $12-$70 per post depending on size, species, and length; $20-$60+ per footing in bagged concrete; $15-$35 per post in connectors; and $200-$400 per post installed if you hire out. The cost guide has full tables.
Related Guides
- Deck Post Complete Guide — the flagship overview: inputs, formulas, and a worked example.
- Deck Post Formulas Explained — derivations with worked numbers in both unit systems.
- Deck Post Material Guide — lumber, steel, connectors, and concrete compared.
- Deck Post Estimating Guide — from plans to a clean material takeoff.
- Deck Post Case Study — a hypothetical 16×20 ft deck, fully estimated and costed.
Get Your Number
Short answers get you oriented; your dimensions get you a materials list. Enter your deck size, beam layout, and footing specs in the Deck Post Calculator for post counts, spacing checks, footing volumes, and post lengths — then use the guides above to verify the details that matter for your site and your code.
Disclaimer: Results are planning estimates only. Verify quantities and prices with your supplier and consult a licensed professional or your local building authority for structural or code-related decisions.
