How Much Concrete for Sonotubes? Volume, Bags & Worked Examples

Planning estimate, not a footing design. Sonotube footings are cylinders, and the volume follows one formula — but diameter follows the post and depth follows the frost line. This guide gives you all three — then run your footings through our sonotube calculator.

The cylinder formula

Volume = π × radius² × depth, with radius in feet. A 12-inch tube is 1 foot across, so the radius is 0.5 feet: a 4-foot-deep tube holds π × 0.25 × 4 = 3.14 cubic feet. Multiply by tube count, add waste for spillage at the top of the pour, and divide by the bag yield — 0.60 cu ft for an 80 lb bag, 0.45 for 60 lb, 0.375 for 50 lb, 0.30 for 40 lb — for the bag count. One 12-inch × 4-foot tube needs 6 bags of 80 lb mix before waste.

Sizing the tube

Tube diameter follows the post: the rule of thumb is a tube at least 3 times the post width. A 6 × 6 post wants a 12-inch tube at minimum; heavier beams, multi-story decks, and corner posts carrying more load push toward 14–16 inches. Common residential sizes: 8 inches for fence posts and light decks, 10–12 inches for deck footings and pergolas, 14–16 inches for heavy decks and corners, 18–24 inches for pole barns and large piers. Spacing follows the beam layout — a 12 × 16 deck might need six footings while a 10 × 10 needs four — not a fixed rule.

Frost depth

Footings must reach below the local frost line, measured from finished grade to the bottom of the tube, not to the top. In the northern US (MI, MN, NY) that is 42–60 inches; in the South (GA, FL, TX) 24–30 inches is typical. Add about 6 inches of gravel at the bottom of the hole. A footing above the frost line heaves in winter and cracks everything it supports — always confirm the exact depth with your local building department, since it varies by jurisdiction and soil.

Bags vs ready-mix

Bagged concrete makes sense under about 1 cubic yard total (roughly 45 eighty-pound bags). Beyond that, ready-mix is usually easier and cheaper per yard, and mixing dozens of bags by hand for a big deck is a long day. Fast-setting mixes set in 20–40 minutes and reach structural strength in 3–4 hours, and yield the same volume per bag as standard mixes — useful for fence posts, but not for large pours where you need working time. The below-grade portion of the tube is left in place; the above-grade portion should be peeled away 24–48 hours after pouring for a cleaner look.

Worked examples

1. Single deck footing, 12 in × 4 ft: 3.14 cu ft — 6 bags of 80 lb (5.24 rounded up), or 8 bags of 60 lb.
2. 6 tubes, 12 in × 3.5 ft, 5% waste, 60 lb bags: 16.49 cu ft × 1.05 = 17.32 cu ft (0.64 cu yd) — 39 bags.
3. 6 tubes, 18 in × 4 ft, 10% waste, 80 lb bags: radius 0.75 ft, π × 0.5625 × 4 = 7.07 cu ft per tube × 6 = 42.41 cu ft × 1.10 = 46.65 cu ft — 78 bags (1.73 cu yd — compare ready-mix at this size).

Frequently asked questions

The rest of the footing job

Footings are one concrete job — for flatwork, use our concrete calculator for slabs and footings by volume, and rebar for the steel inside structural footings.