Deck Post Spacing Calculator
Quick answer: A deck needs a post at both ends of every beam plus one every 6–8 ft in between. Posts = beams × (⌈beam length ÷ spacing⌉ + 1). Worked example: a 16 ft long, 12 ft deep deck attached to the house with a ledger has one beam, so at 8 ft spacing it needs 16 ÷ 8 + 1 = 3 posts; the same deck freestanding has two beams and needs 6. Under the American Wood Council's DCA 6 guide, 4×4 posts are allowed only on low decks — most builders use 6×6 — and posts taller than 14 ft need engineering.
Posts carry the beam down to the footings, and their size depends far more on height than on load. A 4x4 is fine for a low deck and marginal above about six feet, where buckling rather than crushing becomes the limit. This works out the post count, the size your deck height calls for, and the lumber length to buy allowing for trim.
Last updated October 4, 2026 · Sources: American Wood Council, DCA 6 — Prescriptive Residential Wood Deck Construction Guide
Built and maintained by DixonAppStudio LLC · How the figures are worked out · Report an error
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3 posts means 3 footings. Next, get the bags of concrete for them in the footing concrete calculator.
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How far apart should they be? Use the deck post spacing calculator — beam span, post count and footing load.
Adding a pergola? Posts, beams, rafters and concrete are in the pergola materials calculator.
Building a screen? Count lattice panels and posts with the deck privacy screen calculator.
One pier per post: count the concrete for each tube with the Sonotube calculator.
Digging the footings? See our post hole digger buying guide.
How many posts does a deck need?
Count the beams first, then the posts on each. A deck attached to the house with a ledger needs one beam line for every 12 feet or so of joist span — so a 12 ft deep deck has one beam at the outer edge, and a 16 ft deep deck usually has two. A freestanding deck adds a beam where the ledger would have been. Each beam then needs a post at both ends and one every 6 to 8 feet between. A 16 ft long, 12 ft deep attached deck at 8 ft spacing therefore needs 3 posts; the same deck freestanding needs 6.
| Deck (length × depth) | Attached, 8 ft spacing | Freestanding, 8 ft spacing |
|---|---|---|
| 10 × 10 | 3 posts | 6 posts |
| 12 × 12 | 3 posts | 6 posts |
| 16 × 12 | 3 posts | 6 posts |
| 20 × 12 | 4 posts | 8 posts |
| 20 × 14 | 8 posts | 12 posts |
| 24 × 16 | 8 posts | 12 posts |
4×4 or 6×6: what size post for a deck?
Under the American Wood Council's DCA 6 prescriptive guide, a 4×4 is only allowed on low decks, and 6×6 posts are required as height increases, up to a 14 ft limit beyond which the post needs engineering. The reason is buckling, not crushing: a tall, slender post bows sideways long before its wood fails. A 6×6 also leaves enough material to notch for a beam or take a proper post cap, which a 4×4 does not — which is why most builders use 6×6 everywhere. Pricing posts and beams at a yard that sells hardwood or timbers by the board foot? The board foot calculator tallies nominal sizes and lengths into board feet and cost.
Post spacing is really a beam question
How far apart posts can go depends on the beam they carry and the joist span it supports. Bigger beams span further: in Southern pine, a doubled 2×10 carrying 12 ft joists spans about 7 ft between posts, a tripled 2×12 about 9 ft. Wider spacing means fewer posts and footings but a heavier beam. Size the beam for your spacing with the deck beam span calculator.
Footings and post bases
Every post needs its own footing below the frost line — 12 inches across much of the South, 30–42 inches in the Midwest and Mid-Atlantic, 48 inches or more in the North (see your state). Set the post on a galvanized standoff base bolted to the footing rather than burying it in concrete; the gap under the end grain is what keeps it from rotting. Work out concrete with the deck footing calculator.
How much does each post carry?
Decks are designed for 40 psf live load plus about 10 psf dead load. An interior post carries the area halfway to the next post along the beam and halfway back to the ledger — on 8 ft spacing with 12 ft joists that is 48 sq ft, or roughly 2,400 lb. A hot tub changes everything: at 100+ psf the post and footing sizes have to be designed for it, so use the hot tub deck load calculator.
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Frequently asked questions
- Should I use 4x4 or 6x6 deck posts?
- Use 6x6 once the post is more than about six feet from footing to beam, and many builders use 6x6 throughout. The reason is buckling, not strength — a tall slender post fails sideways long before it is crushed. A 6x6 also gives you real material to notch for a beam and enough meat for a proper post cap connection, which a notched 4x4 does not.
- How many deck posts do I need?
- One at each end of every beam plus one every eight feet between, and often every six. A 16 ft beam needs three posts at 8 ft spacing or four at 6 ft. A freestanding deck has two beams, so double it. Add a post or a landing footing at the bottom of the stairs.
- Can deck posts be notched for the beam?
- A 6x6 can be notched to carry a beam, leaving at least the manufacturer's or code's required remaining thickness. A 4x4 should not be — notching it removes too much of a small section and what is left is not enough to carry the beam. The simpler and stronger detail on any post is a rated post cap with the beam bearing on top.
- Do deck posts need to be pressure-treated?
- Yes, and specifically rated for ground contact if the post runs down into or near the soil. Standard above-ground treatment is not enough where wood meets dirt or concrete. Better still, keep the post off the ground entirely on a standoff post base — an inch of air under the end grain is the single cheapest thing you can do for a deck's lifespan.
- How far apart should deck posts be?
- Six to eight feet along each beam is typical. The limit is the beam, not the post: a doubled 2x10 beam carrying 12 ft joists spans about 7 ft between posts in Southern pine under the DCA 6 tables, while a tripled 2x12 reaches about 9 ft. Closer posts let you use a smaller beam; wider spacing means fewer footings but a bigger beam.
- How deep should deck posts be set?
- Posts should not be buried at all in modern practice — they sit on a concrete footing on a post base, and the footing goes below the local frost line. That is 12 inches across much of the South, 30 to 42 inches in the Midwest and Mid-Atlantic, and 48 to 60 inches or more in the northern states. Burying a post in concrete traps water against the end grain and rots it from the inside.
- How much load does a deck post carry?
- Decks are designed for a 40 pound per square foot live load plus about 10 pounds dead load. A post carries the area halfway to its neighbours in each direction — on an 8 ft post spacing with 12 ft joists that is about 48 square feet, or roughly 2,400 pounds. A 6x6 has capacity to spare at normal heights; the footing under it usually governs.
- How many posts do I need for a 12x20 attached deck?
- Four, if the 20 ft side runs along the house. A 12 ft deep attached deck needs one beam at the outer edge, and a 20 ft beam with posts no more than 8 ft apart needs three spans, so four posts. The same deck built freestanding adds a second beam where the ledger would be, for eight posts. Check the beam size for that span in the deck beam calculator before settling on the spacing.
- What are the actual dimensions of a deck post?
- Lumber is sold by nominal size and is smaller in reality: a 4x4 post measures 3 1/2 by 3 1/2 inches and a 6x6 measures 5 1/2 by 5 1/2 inches. Buy post bases and caps for the actual size, and when notching a 6x6 for a beam remember the full post is only 5 1/2 inches across.
- How far apart should porch or railing posts be?
- Porch roof posts are set by the header or beam they carry, so the spacing comes from the beam span, commonly 6 to 8 ft. Guard (railing) posts are a separate job and are normally no more than 6 ft apart for wood railing, closer for cable. Use the deck post spacing calculator for structural posts and the deck railing calculator for guard posts.
