Sonotube & Post Hole Calculator
A round footing is just a cylinder: π × radius² × depth. A 12 inch tube filled 4 ft deep takes 3.14 ft³ of concrete — about 0.12 yd³, or 6 bags of 80 lb premix with a little to spare.
Depth is the concrete depth, not the hole depth — if the post sits in the hole, the concrete around and below it is what you’re pouring. For stepped or belled footings, use the main calculator’s Column shape per section.
Common tube sizes at 4 ft deep
| Diameter | Per hole | Cubic yards | 80 lb bags (exact) |
|---|---|---|---|
| 8 in | 1.40 ft³ | 0.05 yd³ | 3 |
| 10 in | 2.18 ft³ | 0.08 yd³ | 4 |
| 12 in | 3.14 ft³ | 0.12 yd³ | 6 |
| 16 in | 5.59 ft³ | 0.21 yd³ | 10 |
| 18 in | 7.07 ft³ | 0.26 yd³ | 12 |
Hand-dug holes are never the exact diameter on the label — they bell out, slump and vary. The 10% default waste allowance is the minimum for dug holes; rough ground earns 15–20%.
Frequently asked questions
How deep should a fence post footing be?
Common practice puts the bottom of the footing below the local frost line, with the post embedded in the concrete — but frost depth and code requirements vary widely by region. Check your local building department’s fence and deck guidance before you dig.
Should the tube go all the way down?
Tubes are mainly a form for the above-grade and near-surface part, and a way to get a clean, sized hole. Below the tube, the hole walls do the forming — which is exactly why dug holes use more concrete than the cylinder math.
How many bags for six 12 inch deck footings, 4 ft deep?
Each is 3.14 ft³, so six are 18.85 ft³ — 32 bags of 80 lb exactly, 35 with 10% waste. The calculator above runs it for your count.