Deck Post Spacing
Deck joist and post layout
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Working out deck joist and post layout is easier with Deck Post Spacing, which answers as you type. Drop Deck Length (ft), Joist Spacing (in) and Max Beam Span (ft) into Deck Post Spacing — the figure refreshes as you type. In Deck Post Spacing the working sits beside the answer. Deck Post Spacing replaces guesswork. Deck Post Spacing is privacy-first: Home & Garden inputs stay offline. Deck Post Spacing handles deck post spacing calculator joist span lumber the same way as free online deck post spacing calculator — one of the Home & Garden tools on CalcProMaster. Deck Post Spacing has no learning curve. The fields are labeled. Open Deck Post Spacing and enter your numbers.
What the defaults give
A 16 ft deck at 16 inch joist spacing needs 13 joists.
With a maximum beam span of 8 ft, that is 3 posts per row and 6 posts across two rows.
The formula
Joists = ceil(deck length in inches ÷ joist spacing) + 1
Posts per row = ceil(deck length ÷ beam span) + 1
Total posts = posts per row × number of rows
Both counts use the plus-one rule: a 16 foot run of joists at 16 inch centres has positions at 0, 16, 32 … 192 inches, which is 13. The same logic applies to posts along a beam.
Worked examples
16 ft, 16 in joists, 8 ft span → 13 joists, 3 posts per row, 6 total
12 ft, 16 in, 8 ft → 10 joists, 3 per row, 6 total
20 ft, 16 in, 8 ft → 16 joists, 4 per row, 8 total
24 ft, 12 in, 8 ft → 25 joists, 4 per row, 8 total
16 ft, 12 in, 8 ft → 17 joists, 3 per row, 6 total
16 ft, 16 in, 6 ft span → 13 joists, 4 per row, 8 total
Why the span, not the spacing, sets the posts
Joist spacing is about the decking: boards need support at the interval the manufacturer specifies, commonly 16 inches for 5/4 decking and 12 inches for thinner or composite boards running diagonally. Post spacing is about the beam underneath, which carries the floor load to the ground. A longer beam span needs a deeper beam, and the allowable span depends on the timber size and grade, not on a single universal number.
The 8 ft default is reasonable for a built-up 2×8 beam or a pair of 2×10s on a small deck. Check a span table for your species, grade and load — the difference between a 6 ft and 10 ft span can be a beam two sizes larger, and getting it wrong is a structural failure rather than a cosmetic one.
What the layout does not include
Footings are the real constraint. Each post needs a footing sized to the load and the frost depth, and in cold climates that can be 1.2 metres or more deep. The count does not tell you whether the soil can carry the load, whether a permit requires engineered drawings, or whether the deck attaches to the house with a ledger board and the correct flashing.
Add rim joists, blocking between joists at mid-span, hangers, joist tape and stairs to the materials list. A rim joist is not a joist in this count — it is an extra member along each edge and end, and it is what the joists hang from.
Frequently Asked Questions
How many joists for a 16 foot deck?
Thirteen at 16 inch spacing, which is 12 bays plus one. At 12 inch spacing the same deck needs 17.
How many posts under that deck?
Six, arranged as three per row across two rows, when the beam span is limited to 8 feet.
Does joist spacing depend on the decking?
Yes. Composite and thin boards often require 12 inches, while 5/4 timber decking commonly allows 16. Diagonal layouts need closer spacing.
Is an 8 foot beam span always allowed?
Not universally. It depends on timber size, species and grade, and on the load. Check a span table or an engineer before building.
What is missing from the count?
Footings, rim joists, blocking, hangers, stairs and the ledger connection. The count covers joists and posts only.
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