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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.
With these inputs:
| Result | Value |
|---|---|
| Posts needed | 3 posts |
| Post size | 4×4 allowed, 6×6 recommended |
| Beam lines | 1 |
| Posts per beam | 3 |
| Actual post spacing | 8.0 ft |
| Tributary area per interior post | 48 sq ft |
| Design load per interior post | 2,400 lb |
| Footings (one per post) | 3 footings |
| Buy each post as | 8 ft stock |
DCA 6 permits 4×4 posts only on low decks; 6×6 gives room for a proper beam connection. Add a landing pad or footing at the bottom of any stairs. Footing depth follows your local frost line.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
Need the whole roof at once? The free roofing calculator works out roof area, squares, shingle bundles, underlayment, drip edge, ridge cap and installed cost from one set of measurements.