Deck Post Spacing Calculator
Evenly spaced posts keep a deck beam strong and code-compliant. Enter the beam length and your maximum allowed spacing to get the post count and exact on-center layout.
Most codes limit post spacing to 8 ft for typical residential decks, but always confirm with your local building department — soil, beam size, and load change the rules.
How the deck post spacing calculator works
Dividing the beam length by your maximum spacing and rounding up gives the number of spans; adding one gives the post count. The calculator then re-divides the length by the actual number of spans so every gap is identical — a 20 ft beam at 8 ft maximum becomes 4 posts at exactly 6 ft 8 in on-center.
Even spacing is not just cosmetic: it spreads the load uniformly so no single span is overstressed. End posts sit at the beam ends, and interior posts divide the rest equally.
spans = ⌈length ÷ max⌉ · posts = spans + 1 · on-center = length ÷ spans
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Deck post spacing calculator FAQ
How far apart should deck posts be?
Common practice and many codes cap spacing at 8 feet on-center for residential decks. Longer spans need bigger beams — check local code and span tables for your lumber size.
What does “on-center” mean?
On-center spacing is measured from the center of one post to the center of the next, not edge to edge. Mark your layout with these center points.
Do the end posts count in the total?
Yes. The calculator includes a post at each end of the beam, then spaces the interior posts evenly between them.
How deep should deck post footings be?
Footings must sit below the local frost line — commonly 36–48 inches in cold climates. Use the post depth logic from our fence post depth calculator as a starting point.
Can I space posts wider than 8 feet?
Only with an engineered beam sized for the longer span and local approval. For standard dimensional lumber, stay at 8 ft or less.