How Far Apart Should Deck Posts and Beams Be?
How the load flows
Every question about post spacing is really a question about the beam. Deck load runs boards → joists → beam → posts → footings, so the distance a beam may span between posts depends on how much deck each foot of beam is carrying — which is set by the joist span feeding it. A beam under 12 ft joists carries half again the load of the same beam under 8 ft joists, and its allowed span shrinks accordingly.
Typical beam spans between posts
| Beam (doubled 2x, treated) | Joists span 8 ft | Joists span 10 ft | Joists span 12 ft |
|---|---|---|---|
| (2) 2x8 | ~6.5 ft | ~6 ft | ~5.5 ft |
| (2) 2x10 | ~8 ft | ~7 ft | ~6.5 ft |
| (2) 2x12 | ~9 ft | ~8 ft | ~7.5 ft |
Read the table the practical direction: on a common 16 × 12 ft deck — the deck calculator's worked example — with joists spanning 12 ft to a beam along the 16 ft side, a doubled 2x10 wants posts about every 6.5 ft: three posts, one mid-span. Upgrade the beam to a doubled 2x12 and two end posts plus one centre still work with margin at 8 ft spacing.
Posts: use 6x6
A 4x4 post is permitted only for low, short decks in some jurisdictions and prohibited outright in others — it can also not be properly notched to seat a doubled beam. A 6x6 costs a few dollars more per post, notches cleanly so the beam bears on wood rather than hanging on bolts, and removes the question. Whatever the size, the connections do the real work: post base anchored to the footing, beam seated in a notch or a listed post cap — never toenails.
Footings: one per post, below frost
Each post lands on a concrete footing that bears below the local frost line, same logic as a fence post but carrying far more load — typical diameters run 12 to 24 in depending on soil and tributary area. A 12 in diameter footing 36 in deep is about 2.4 cu ft of concrete, four 80 lb bags; size the whole set with the concrete calculator's column mode before deciding between bags and a short-load delivery.
Cantilevers buy flexibility
The beam does not have to sit at the end of the joists: joists may typically cantilever past the beam by up to about one-quarter of their back span. Pulling the beam in 2 ft on a 12 ft deck shortens the effective joist span, which lightens the beam load, which stretches the allowed post spacing — and hides the posts in shadow instead of at the deck edge. It is the cheapest structural trick in deck design, and it is still one to confirm against the same code table.
Enter the deck size and your board width to get the board count, the linear footage to order, the joists underneath and the screws to fix it all down — with the gap between boards included, which is what most quick estimates forget.
Frequently asked questions
How many footings does a 16x12 deck need?
Usually three along one beam — posts at roughly 8 foot centres — plus the ledger carrying the other end of the joists. A freestanding version needs a second beam and three more footings.
Can I use 4x4 posts for a deck?
Only for low, small decks where local code still allows it, and a doubled beam cannot be notched into a 4x4 properly. A 6x6 is the practical standard and many jurisdictions now require it.
How far can joists cantilever past the beam?
Commonly up to one-quarter of the joist back span, so a joist spanning 8 feet to the beam may overhang about 2 feet. The exact limit is in the code joist table for your lumber and spacing.
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