Concrete per tube, total yards, and 80lb bags
Sonotube — the round fiber form tube brand from Sonoco that has become the generic name for concrete form tubes — sets round piers for decks, porches, pergolas, fence gates, and sign posts. The only math involved is the volume of a cylinder, V = π r² h, but multiplying by hand across a dozen piers of different depths invites errors, and errors in concrete mean either a short pour or wasted mix. This calculator does the cylinder volume for each tube size and depth, converts to cubic yards or 80-lb bag equivalents, and prices the pour.
Enter the tube diameter in inches, the pour depth in feet, and the number of piers. The calculator computes volume per tube, total volume, bags of premix at roughly 0.60 cubic feet per 80-lb bag of concrete, and total cost using your bag or ready-mix price. For decks in climates with frost, tube depths are usually set by the local frost line, so check your code before committing to a depth.
Two field realities adjust the raw number. Tubes set in belled or flared footings (some plastic footing forms flare at the bottom) hold more concrete than a straight cylinder, and most crews order or mix a small cushion of extra concrete — commonly 5 to 10 percent — for spillage, tube settlement, and the domed top many builders pour above grade.
Ordering tip: for multiple piers of the same size, calculate one tube then multiply by the count, and always round bags up — leftover mix beats a short pour mid-finish.
Volume in cubic feet = π × (diameter ÷ 2)² × height in feet. A 12-inch tube poured 4 feet deep is π × (0.5 ft)² × 4 = 3.14 cubic feet. In cubic yards — the unit ready-mix plants sell by — divide cubic feet by 27, so that single tube is about 0.116 yd³. Bag math follows the same volume rule: an 80-lb bag of standard premix yields approximately 0.60 cubic feet of mixed concrete, so 3.14 ÷ 0.60 ≈ 5.2, call it 6 bags per tube.
Weight matters for small pours too. Concrete weighs roughly 145 to 150 pounds per cubic foot when mixed, so that 12-inch × 4-foot pier weighs close to 470 lb. This is why mixed-by-bag piers above roughly a cubic yard usually shift to ready-mix or a small-load delivery — a standard bag-a-minute mixer pace cannot keep up with a pour that size.
The table below shows the volume per foot of depth for the standard retail tube diameters, along with 80-lb bag equivalents per foot of pour. Numbers use the straight cylinder formula and round bags up, since you cannot mix a fraction of a bag.
Diameter choice is usually structural: 8-inch and 10-inch tubes serve fence posts and light loads, 12-inch is the common default for deck piers on decent soil, and larger diameters or multiple piers appear where soil bearing is weak or loads are heavy. If an engineered plan specifies the pier size, follow the plan — the tables are a convenience for the typical residential case, not a substitute for a designed footing.
| Tube diameter | Volume per ft of depth | 80-lb bags per ft | Typical use |
|---|---|---|---|
| 8 in | 0.35 cu ft | 0.6 | Fence posts, small signs |
| 10 in | 0.55 cu ft | 0.9 | Light deck piers |
| 12 in | 0.79 cu ft | 1.3 | Standard deck piers |
| 14 in | 1.07 cu ft | 1.8 | Heavier deck loads |
| 18 in | 1.77 cu ft | 3.0 | Pergolas, porch columns |
For cost, multiply total bags by your local bag price, or multiply total cubic yards by the ready-mix price plus short-load fees, which many plants add below about 1 yard. Add the cushion before comparing prices: a 10 percent overage on a 20-tube deck job is one extra batch, not a second delivery, so most estimators deliberately round bags up to full pallet quantities where discounts kick in.
How much concrete fits in a 12-inch Sonotube per foot?
A 12-inch diameter tube holds about 0.79 cubic feet per foot of depth — π × (0.5 ft)² × 1 ft. That is roughly 1.3 eighty-pound bags of premix per foot of pour.
How many 80-lb bags is one 12-inch tube 4 feet deep?
3.14 cubic feet ÷ 0.60 cubic feet per bag ≈ 5.2 bags, so plan on 6 bags per tube to be safe. Mixing slightly short is the classic mistake that leaves a low pier.
How do I convert Sonotube volume to cubic yards?
Compute cubic feet with π × (diameter ÷ 2)² × depth, then divide by 27. Twenty 12-inch tubes at 4 feet total about 2.33 cubic yards, before any waste cushion.
How deep should Sonotube piers be?
In frost climates, code typically requires the pier bottom at or below the local frost line, which varies widely by region. Always confirm the required depth with your local building department or engineered plans.
Should I order ready-mix or use bags?
Bag mixing is economical up to roughly 1 cubic yard total. Beyond that, the labor and consistency risk usually make a small ready-mix load — even with short-load fees — the better choice.