How Much Concrete Do I Need for a Sonotube?
A standard 12-inch sonotube filled 4 feet deep takes about 3.46 cubic feet of concrete, roughly 6 bags of 80-lb mix or 8 bags of 60-lb mix once you add a 10% waste allowance. Smaller 8-inch tubes need as few as 3 bags per 4 feet of depth, while a wide 24-inch column can take 23 bags or more. Below is the formula, a bag count by diameter and depth, and a calculator that gives the exact number for your tube.
Key takeaways
- A 12-inch sonotube 4 feet deep needs about 6 bags of 80-lb concrete mix, or 8 bags of 60-lb mix, with 10% waste included.
- Concrete volume scales with the radius squared, so going from a 10-inch to a 12-inch tube adds roughly 45% more concrete at the same depth.
- A 12-inch tube uses about 1.4 bags of 80-lb mix per linear foot of depth, so a 6-foot tube needs roughly 9 bags.
- Not every sonotube needs a separate footing pad, but codes commonly require one for decks and porches, especially in sandy or loose soil.
- One 80-lb bag of concrete yields 0.60 cubic feet, so it takes about 45 bags to fill a full cubic yard.
The formula: how much concrete fills a sonotube
A sonotube is a cylinder, so its volume is pi times the radius squared times the depth, all in feet. Since diameter is usually given in inches, divide it by 24 to get the radius in feet, then square that number before multiplying by pi and the depth.
Radius (ft) = Diameter (in) ÷ 24Volume (ft3) = π x radius2 x depthFor a 12-inch tube, the radius is 0.5 feet. At 4 feet deep that is pi times 0.25 times 4, about 3.14 cubic feet. Add 10% for spillage and an uneven hole bottom and you land at roughly 3.46 cubic feet, which rounds up to 6 bags of 80-lb concrete mix. For any diameter and depth, the sonotube calculator runs this math and returns the bag count directly.
Concrete per linear foot, by sonotube diameter
Because volume depends on the radius squared, a small jump in diameter adds a lot of concrete. The table below shows how much an extra foot of depth costs you in 80-lb bags, already including a 10% waste allowance, so you can scale to any hole depth without redoing the formula.
| Tube diameter | Cubic feet per ft (+10%) | 80 lb bags per ft |
|---|---|---|
| 8 in | 0.38 | 0.64 |
| 10 in | 0.60 | 1.00 |
| 12 in | 0.86 | 1.44 |
| 16 in | 1.54 | 2.56 |
| 18 in | 1.94 | 3.24 |
| 24 in | 3.46 | 5.76 |
Multiply the bags-per-foot column by your actual hole depth and round up. A 10-inch tube 5 feet deep, for example, needs about 5 bags, while the same depth in a 16-inch tube jumps to about 13 bags.
How many bags for a 12-inch sonotube, by depth
The 12-inch tube is the most common size for deck posts and pergola footings, so here is the bag count at typical hole depths, with a 10% waste allowance already built in and rounded up to the next whole bag.
| Depth | Cubic feet (+10%) | 80 lb bags | 60 lb bags |
|---|---|---|---|
| 2 ft | 1.73 | 3 | 4 |
| 3 ft | 2.59 | 5 | 6 |
| 4 ft | 3.46 | 6 | 8 |
| 5 ft | 4.32 | 8 | 10 |
| 6 ft | 5.18 | 9 | 12 |
| 8 ft | 6.91 | 12 | 16 |
Depth is usually set by the local frost line rather than the post load, so measure the actual hole before buying rather than assuming the common 4-foot figure applies everywhere.
Does a sonotube need a separate footing pad?
Not always. A straight sonotube set below the frost line is often enough for light structures like fences, mailboxes and playsets on stable soil. The tube itself acts as the footing once it cures.
For structural loads like a deck or porch, most codes require a wider footing pad or flared base at the bottom of the tube to spread the load into the soil, especially where the ground is sandy or loose. Footings must also sit at least 12 inches below grade and below the local frost line, whichever is deeper, per IRC R403.1.4.
If your project is a deck or porch pier rather than a simple post hole, check the load and footing size requirements in how much concrete for a deck footing before you dig, since the pad adds concrete beyond the tube volume above.
Bags versus ready-mix: what it costs in 2026
In 2026, an 80-lb bag of standard concrete mix runs about 8 to 9 dollars at most home centers, with sonotube forms adding roughly 12 dollars for an 8-inch tube up to 30 to 40 dollars for wider or longer ones. A single 12-inch tube 4 feet deep typically lands around 50 to 70 dollars in material, form included.
Once a project needs more than about 8 to 10 tubes this size, the concrete adds up to roughly a full cubic yard, and delivered ready-mix, which runs about 125 to 180 dollars per cubic yard, often beats hauling 40 or more bags by hand.
If the same project also involves setting posts without a tube, like a fence line, the math changes slightly since the post itself displaces some volume. The breakdown in bags of concrete per fence post covers that adjustment.
Tips to avoid running short mid-pour
Measure the actual tube diameter and hole depth before buying, not the nominal size on the label. Sonotube forms sold as 12 inches can run slightly larger inside, and a hand-dug hole is rarely a perfect cylinder.
Always round every bag count up, never down. A tube running short with wet concrete already curing in the mix cannot wait for another trip to the store.
Buy one extra bag beyond the calculated total for any tube deeper than about 4 feet, since deeper pours lose more concrete to an uneven hole bottom and settling as the form fills.
Frequently asked questions
How many bags of concrete do I need for a sonotube?
How many 80 lb bags of concrete to fill a 12 inch sonotube?
How much Quikrete for 12 Sonotube?
Does a 12 sonotube need a footing?
How much concrete do I need for an 8 inch sonotube?
References
- Concrete (Wikipedia)
- Sonotube Concrete Forms (Sonoco manufacturer)
- QUIKRETE Concrete Mix (product data)
- IRC 2021 R403.1.4 Minimum Depth (ICC Digital Codes)