Deck Footing Calculator
Plan a deck's footings from its size: how many footings on the standard 8 ft grid, and the concrete to fill them at your tube size and frost depth.
Deck footings on the standard grid
Deck footings follow a planning grid: posts no more than about 8 ft apart in either direction, so beams and joists of ordinary sizes can carry their share without heroics. The count is the grid counted honestly — positions along the deck's length, rows across its width, multiplied. A freestanding deck stands on all its own footings; a ledger-attached deck lets the house carry the wall side, so that row comes off the count (the ledger and its flashing become the critical detail instead).
Positions = deck length ÷ 8 ft, rounded up, + 1 · Rows = width ÷ 8 ft, rounded up, + 1 (minus the house row if ledger-attached) · Concrete = footings × tube volume
Example: a freestanding 16 × 12 ft deck: 3 positions by 3 rows = 9 footings. In 12 in tubes dug 4 ft, that is 28.27 cu ft of concrete — 52 bags of 80 lb mix (70 of 60 lb) with the allowance.
Read this as the standard planning layout it is: real footing counts and sizes are governed by local code, your soil's bearing capacity, and the deck's actual beam and joist layout — a big deck, a hot tub, or a roof over the deck all push loads past what a rule-of-thumb grid assumes, and footing diameters in particular are a code table, not a guess. Confirm the layout locally before you dig. Depth follows the frost line: footings must bottom out below it, so set the depth from your local requirement. The sonotube calculator gives the same concrete count tube by tube if your footings vary in size.
Frequently asked questions
How far apart should deck footings be?
The planning grid used here is about 8 ft in both directions, which suits typical beam and post sizes for a residential deck. Tighter grids (6 ft) suit heavy loads or smaller beams; wider spans need bigger beams and an engineered look. Your local code's footing table — driven by soil and tributary area — has the final say.
Does a ledger-attached deck need footings along the house?
No — the ledger board, bolted into the house's rim joist, carries that side, and the footing rows start out in the yard. What it does need is correct ledger flashing and fasteners; more decks fail at a rotten, under-fastened ledger than at any footing. Freestanding decks skip the ledger entirely and stand on a full grid instead.
What size footing does a deck need?
Diameter follows the load each footing carries and the soil under it — 12 in tubes are the common residential default, 8–10 in serve small low decks on good soil, 16 in and up appear under big tributary areas or weak soil. It is a local code table lookup once you know the deck's layout, not a universal number.
Can deck footings sit on top of the ground?
In most places, no — frost heave will walk them, and codes ask for embedment below the frost line or an approved frost-protected design. Floating deck blocks exist for small, low, freestanding decks in some jurisdictions; whether yours allows them is a local question. When in doubt, dig.
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