What sonotubes are
Sonotubes are round cardboard form tubes — the standard way to pour concrete piers for decks, posts, and light structures. You set the tube in a dug hole, fill it with concrete, and set your post bracket in the wet mix. Getting the volume right matters because piers are usually a multi-unit job: one wrong estimate multiplied by twelve piers is a real problem.
Common tube diameters
Residential deck piers typically use 8, 10 or 12-inch diameter tubes; heavier loads go larger. Diameter dominates the math — doubling the diameter quadruples the volume, so measure carefully. Height is the tube length above and below grade combined.
The math: πr²h
Round columns use cylinder volume: π × radius² × height, all in feet. A 12-inch tube, 4 feet tall: radius 0.5 ft, so 3.1416 × 0.25 × 4 = 3.14 ft³ = 0.12 yd³, about 6 eighty-pound bags (3.14 ÷ 0.60 = 5.2, rounded up). The calculator handles the radius conversion for you.
Piers plus a slab
Deck jobs often pair piers with a small slab or footing pads. Enter each pier once, tap “Add this pour to project,” then add the slab — the project total gives you one order number for the plant or one bag count for the store run.
Frequently asked questions
How many bags of concrete per sonotube?A 12-inch diameter tube, 4 feet tall takes about 6 eighty-pound bags (3.14 ft³ ÷ 0.60). An 8-inch tube of the same height takes about 3 bags. Enter your exact size above for the precise count.
What size sonotube do I need for a deck?10 or 12-inch tubes are typical for residential deck posts; 8-inch suits light-duty posts. Diameter is structural — follow your deck plan or local code.
How deep should pier footings go?Below the local frost line, same as strip footings — 12 inches in warm climates, 36–48+ inches up north. Check local code; depth is structural, not a volume decision.
How much concrete for 6 deck piers?Six 12-inch × 4-foot piers: 6 × 3.14 ft³ = 18.8 ft³ = 0.70 yd³, about 32 eighty-pound bags before waste. That's over the half-yard rule, so price a small ready-mix load against the bags.
More concrete calculators
Disclaimer: This calculator provides estimates for planning purposes only — not professional engineering advice. Always verify quantities with your supplier and follow local building codes for footings, structural walls and load-bearing work. Prices shown are 2026 estimates; get a local quote.