Deck Post Spacing Calculator
Post spacing is set by the beam: a bigger beam spans further between posts, and a longer joist span puts more weight on it, which pulls the posts closer together. This reads the beam span table for your beam and joist length, lays out the fewest posts that stay inside it (letting the beam overhang its end posts if you want), and works out the load each footing carries so you can size the footings to match.
What you'll need
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What decides how far apart deck posts go
Posts hold up the beam, so the distance between posts is simply the span of the beam. A beam's allowable span depends on its size and on how much weight lands on it — and the weight depends on how far the joists span onto it. That is why the same doubled 2×10 can have posts nearly 10 ft apart under 6 ft joists but under 7 ft apart under 14 ft ones. Post size (4×4 or 6×6) does not change the spacing; it is about height and buckling.
Deck post spacing chart
Maximum distance between post centres (southern pine No. 2, 40 psf live + 10 psf dead — the same table as our beam span chart):
| Beam | 6 ft joists | 8 ft joists | 10 ft joists | 12 ft joists | 14 ft joists |
|---|---|---|---|---|---|
| 2-ply 2×6 | 6 ft 8 in | 5 ft 11 in | 5 ft 4 in | 4 ft 10 in | 4 ft 6 in |
| 2-ply 2×8 | 8 ft 6 in | 7 ft 6 in | 6 ft 9 in | 6 ft 2 in | 5 ft 9 in |
| 2-ply 2×10 | 9 ft 8 in | 8 ft 10 in | 8 ft | 7 ft 5 in | 6 ft 10 in |
| 2-ply 2×12 | 11 ft 2 in | 10 ft 3 in | 9 ft 4 in | 8 ft 7 in | 8 ft |
| 3-ply 2×8 | 10 ft 6 in | 9 ft 5 in | 8 ft 6 in | 7 ft 9 in | 7 ft 2 in |
| 3-ply 2×10 | 12 ft 5 in | 11 ft 1 in | 10 ft | 9 ft 2 in | 8 ft 6 in |
| 3-ply 2×12 | 14 ft 1 in | 12 ft 9 in | 11 ft 8 in | 10 ft 8 in | 9 ft 11 in |
Douglas fir runs about 5% shorter, hem-fir and spruce-pine-fir about 10% shorter. Joists longer than 14 ft need a second beam line or an engineered beam.
From spacing to post count
Divide the beam length by the maximum spacing and round up to get the number of bays; add one for the posts. A 16 ft beam with a 7 ft 5 in limit needs three bays — four posts about 5 ft 4 in apart — if the posts sit at the very ends. Let the beam overhang each end post by up to a quarter of the spacing and it drops to two bays: three posts about 6 ft 5 in apart, with 1 ft 7 in of beam past each end post. That is usually the better build — one less footing, and the posts are tucked out of sight.
Load per post and footing size
Each middle post carries the deck area halfway to the next post on either side and halfway back to the ledger — its tributary area — at 50 lb per square foot. With 12 ft joists and posts 8 ft apart (a 3-ply beam) that is 6 × 8 = 48 sq ft, or 2,400 lb. Divide by the soil's bearing capacity (1,500 psf if you do not know it) for the footing area: 1.6 sq ft, which is an 18 in round footing. Closer posts mean smaller footings; the calculator does the arithmetic, and the deck footing calculator turns it into bags of concrete.
Common mistakes
- Spacing posts by eye at "about 8 ft" without checking the beam and joist span.
- Notching a 4×4 or 6×6 post to carry a beam on a shelf — use a post cap or let the beam bear fully on a notched 6×6 as DCA 6 details.
- Putting footings above the frost line; see the deck footing depth guide.
- Forgetting that a freestanding deck needs a second beam — and a second row of posts — where the ledger would be.
For post size by height use the deck post calculator; for beam sizing on its own, the deck beam span calculator.
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Frequently asked questions
- How far apart should deck posts be?
- Most decks end up with posts 6–8 ft apart. The limit comes from the beam: a doubled 2×10 carrying 12 ft joists can span about 7 ft 5 in between posts, a tripled 2×12 about 10 ft 8 in. Longer joists put more load on the beam and pull the posts closer.
- What is the maximum span for a 4×4 or 6×6 post?
- Post size does not set the spacing — the beam does. Post size is about height: 6×6 is the usual choice and required by many jurisdictions, while 4×4 posts are limited to short decks. Use the deck post calculator to check post size for your height.
- How many posts does a 12×16 deck need?
- An attached 12×16 deck with a doubled 2×10 beam carrying 12 ft joists needs three posts along the 16 ft beam if the beam overhangs the end posts — about 6 ft 5 in apart with 1 ft 7 in of overhang at each end — or four posts set right at the ends. A freestanding version doubles that, with a row under each beam.
- How big should deck footings be?
- Divide the load on the post by the soil bearing capacity. A middle post carrying 2,400 lb on 1,500 psf soil needs about 1.6 sq ft of footing — an 18 in round footing. Stronger soil or closer posts allow smaller footings.
