A deck post that carries 52 sq ft of deck needs an 18 in round footing (or 16 in square) at least 6 in thick on ordinary clay soil, set at least 12 in deep and below the frost line. With a 10 in tube pier up to grade from 36 in down, that is 2.25 cu ft of concrete, or 5 bags of 80 lb mix per hole with 10% extra.
How deck footings are sized
A footing is sized by the deck area it holds up, its tributary area. Each joist hands half its span to the beam, plus everything past the beam if it cantilevers, and each post takes the length of beam halfway to the next post on each side. An end post takes only half that.
Tributary area = (joist span ÷ 2 + cantilever) × beam span between posts (÷ 2 for an end post)
IRC Table R507.3.1 turns that area and the soil's bearing value into a minimum footing. Values between rows are interpolated, as the table allows, and rounded up to the whole inch. Depth is the larger of your frost depth and 12 in (R507.3.2 and R507.3.3).
Worked example: middle post of a 16 × 12 ft deck
Joists span 11 ft onto the beam with 12 in of overhang, and posts stand 8 ft apart. The middle post carries (11 ÷ 2 + 1) × 8 = 52 sq ft. Between the 40 and 60 sq ft rows for 1,500 psf soil the table gives 17.8 in round, so the footing is 18 in round, or 16 in square, and 6 in thick.
With a 36 in frost depth:
- Footing pad: π × (9 in)² × 6 in = 0.88 cu ft
- Tube pier: π × (5 in)² × 30 in = 1.36 cu ft
- Per footing: 2.25 cu ft, 5 bags of 80 lb (2.25 × 1.1 ÷ 0.60)
- Three footings: 6.74 cu ft, 13 bags in one order
If you dig an 18 in hole and fill it all the way up, each footing takes 5.3 cu ft, more than twice as much. The end posts of the same beam carry 26 sq ft and could shrink to 14 in round; the calculator on the deck page sizes every footing for the heaviest post, which keeps the dig simple.
Minimum footing size (IRC Table R507.3.1, 40 psf live load)
| Tributary area | 1,500 psf: square / round / thick | 2,000 psf: square / round / thick | 3,000 psf: square / round / thick |
|---|---|---|---|
| 5 sq ft | 7 / 8 / 6 in | 7 / 8 / 6 in | 7 / 8 / 6 in |
| 20 sq ft | 10 / 12 / 6 in | 9 / 9 / 6 in | 7 / 8 / 6 in |
| 40 sq ft | 14 / 16 / 6 in | 12 / 14 / 6 in | 10 / 12 / 6 in |
| 60 sq ft | 17 / 19 / 6 in | 15 / 17 / 6 in | 12 / 14 / 6 in |
| 80 sq ft | 20 / 22 / 7 in | 17 / 19 / 6 in | 14 / 16 / 6 in |
| 100 sq ft | 22 / 25 / 8 in | 19 / 21 / 6 in | 15 / 17 / 6 in |
| 120 sq ft | 24 / 27 / 9 in | 21 / 23 / 7 in | 17 / 19 / 6 in |
| 140 sq ft | 26 / 29 / 10 in | 22 / 25 / 8 in | 18 / 21 / 6 in |
| 160 sq ft | 28 / 31 / 11 in | 24 / 27 / 9 in | 20 / 22 / 7 in |
The thickness applies to plain concrete. Above 160 sq ft the table stops; add a post or have the footing designed. Snow-load rows (50 to 70 psf) are built into the calculator.
Which soil value to use
IRC Table R401.4.1(1) gives presumptive values: 1,500 psf for clay and silt, 2,000 psf for sand and silty or clayey gravel, 3,000 psf for sandy gravel and gravel. Use 1,500 psf unless you know better; it is the lowest value in the table, and where the building official expects anything weaker the code asks for a soil test. On sand the same 52 sq ft post needs only a 16 in footing, on gravel 14 in.
How deep deck footings go
At least 12 in below undisturbed ground, and below the frost line where the deck is attached to a house that is protected from frost. Frost depth is set by your building department for your town, so the calculator asks for it rather than guessing. AWC's DCA 6 adds that footings closer than 5 ft to the house foundation should bear at the same level as the house footing.
Tubes and post bases
A round tube form keeps the pier neat and saves concrete. It must still give the post complete bearing (footnote c): the corners of a 6×6 reach 7.8 in across, so an 8 in tube is the smallest that works and a 10 in tube leaves room for the post base. Posts need lateral restraint too: a manufactured post base, or 12 in of embedment (R507.4.1). Use galvanized (ZMAX) or stainless connectors with treated lumber.
Next steps
Pour volumes for other tube sizes and heights are in the Sonotube concrete calculator, and bag or ready-mix totals for the whole job in the concrete calculator. For wall and strip footings under a house, use the concrete footing calculator. The beam span that sets your post spacing comes from the deck joist span calculator, and the deck calculator sizes every footing of the deck at once.