Deck Footing Count Calculator
Work out how many footings and posts a deck needs from its size and the maximum post and beam spacing.
How it works
beams = ceil(deck width ÷ max beam spacing) · posts per beam = ceil(deck length ÷ max post spacing) + 1 · footings = beams × posts per beam
A house-attached deck hangs from a ledger board on the inner edge and stands on posts along one or more beams. The number of beam rows comes from the width divided by how far the joists can span between supports; the posts along each beam come from the length divided by the maximum post spacing, plus one for the end. Each post lands on its own footing, so multiply the two.
Worked example
A 16 × 12 ft deck with 8 ft max post spacing and 8 ft max beam spacing: beams = ceil(12 ÷ 8) = 2 rows, posts per beam = ceil(16 ÷ 8) + 1 = 3, so 2 × 3 = 6 footings. The ledger on the house supports the inner edge, so the outer beam and a mid beam carry the rest.
Frequently asked questions
Does the ledger count as a support?
Yes — a properly lag-bolted or through-bolted ledger carries the inner edge of the deck, which is why the footings only support the beams standing away from the house. A freestanding deck with no ledger needs a beam and its own footings on all sides.
What sets the post spacing?
The beam size and species set how far it can span between posts — commonly 6 to 8 feet for typical built-up beams — while the joist size sets how far beams can be apart. Both come from the deck span tables in the residential code, so confirm your spacings there.
How big should each footing be?
The footing diameter and depth depend on the tributary load on that post and the soil’s bearing capacity, and the base must sit below the frost line. Once you have the count here, size each footing’s concrete with a footing calculator and your local frost depth.
Related calculators
Estimates only — add for waste and confirm every load-bearing decision against local building codes and a design professional.