Reference
Deck footing sizes and spacing
A footing has two sizes and people usually only think about one. The tube sets how much concrete you pour; the pad at the bottom is what actually stops the deck sinking, and it is sized by load and soil, not by the post above it. Here is both, plus the frost rule that decides how deep you dig.
Form tube sizes and concrete per foot
A form tube is a cylinder, so the concrete it takes is fixed by its diameter and how deep you go. These figures are per foot of depth — multiply by your dig and by the number of piers.
| Tube | Per foot of depth | 80 lb bags per foot | Suits |
|---|---|---|---|
| 6 in | 0.20 ft³ | 0.33 | Light posts, mailbox, small pergola |
| 8 in | 0.35 ft³ | 0.58 | The usual minimum under a 4×4 deck post |
| 10 in | 0.55 ft³ | 0.91 | 6×6 posts, standard residential deck |
| 12 in | 0.79 ft³ | 1.31 | 6×6 posts carrying a longer beam span |
| 16 in | 1.40 ft³ | 2.33 | Heavy beams, roof loads over the deck |
| 18 in | 1.77 ft³ | 2.95 | Structural piers, poor soil |
A 10 in tube at a 42 in frost depth is 3.5 ft of pour, so 1.9 cubic feet, or about four 80 lb bags per pier. Over twelve piers that is a mixer job, not a wheelbarrow job.
Pad size by load, not by post
The pad at the bottom spreads the load into the soil. Size it from the tributary area — the patch of deck each post carries, which is half the joist span by half the beam span on each side — at a residential design load of 50 lb per square foot, and divide by what the soil can take.
pad area = load ÷ soil bearing
diameter = √(4 × pad area ÷ π)
| Tributary area | Load | Pad on 1 500 psf soil | Pad on 2 000 psf soil |
|---|---|---|---|
| 24 ft² | 1 200 lb | 0.80 ft² — 12 in round | 0.60 ft² — 11 in round |
| 36 ft² | 1 800 lb | 1.20 ft² — 15 in round | 0.90 ft² — 13 in round |
| 48 ft² | 2 400 lb | 1.60 ft² — 17 in round | 1.20 ft² — 15 in round |
| 64 ft² | 3 200 lb | 2.13 ft² — 20 in round | 1.60 ft² — 17 in round |
| 80 ft² | 4 000 lb | 2.67 ft² — 22 in round | 2.00 ft² — 19 in round |
Notice how quickly the pad outgrows the tube. A 10 in tube on a 36 ft² tributary wants a 15 in pad under it — which is why a bell footing, or a square pad poured under the tube, is standard rather than exotic. Soil bearing of 1 500 psf is the usual presumptive value for ordinary soil; sand and gravel take more, soft clay takes far less.
Which tube for a 4×4 or a 6×6 post
| Post | Actual size | Minimum tube |
|---|---|---|
| 4×4 | 3.5 in | 8 in |
| 6×6 | 5.5 in | 10 in |
| 8×8 | 7.25 in | 12 in |
The post does not sit in the concrete on a deck — it sits on a galvanised base anchored to the top of the pier, clear of standing water. The tube only has to be wide enough to take that base and its bolt.
Frost depth decides the dig
A footing that stops above the frost line gets lifted by the ground freezing under it, and it does not go back down evenly. The bottom of the footing has to sit below the local frost depth — a figure your building department publishes, and the only number on this page you cannot work out yourself.
| Region | Typical frost depth | 10 in tube takes |
|---|---|---|
| Gulf coast, southern Florida | 0–12 in | under 1 bag |
| Mid-Atlantic, Pacific Northwest | 18–24 in | 2 bags |
| Midwest, New England | 36–48 in | 3–4 bags |
| Northern plains, interior Alaska | 60–100 in | 5–8 bags |
Setting posts in concrete instead of on piers? That is the post hole calculator. For the bag yields behind every figure here, see concrete bag yields and mix ratios, and for the steel in a larger pier, rebar sizes and weights.