How this calculator works
Each column is a cylinder, so its volume is π × radius² × height. The radius is half the diameter converted to feet. The calculator multiplies by the number of columns, adds waste and divides by the bag yield.
Radius (ft) = Diameter (in) ÷ 24
Per column = π × Radius² × Height (ft)
Total = Per column × Count × (1 + Waste% ÷ 100)
Bags = round up( Total ÷ bag yield )
Assumptions and tips
- Bag yields follow the manufacturer’s published figures: 80 lb ≈ 0.60 cu ft, 60 lb ≈ 0.45 cu ft, 50 lb ≈ 0.375 cu ft, 40 lb ≈ 0.30 cu ft.
- Bag counts are always rounded up to whole bags.
- The waste allowance covers spillage, uneven subgrade and over-excavation. 5–10% is common; use more for irregular forms.
- For a fence or deck post set in concrete, the post takes up some space. Use the Fence Calculator if you want the post volume subtracted.
Worked example
Using the default values in the calculator above:
| Input | Value |
|---|---|
| Diameter | 12 in |
| Height or depth | 4 ft |
| Number of columns/holes | 4 |
| Waste allowance | 5 % |
| Bag size | 80 lb bag (0.60 cu ft) |
| Result | Value |
|---|---|
| Total concrete | 0.49 cubic yards |
| 80 lb bags to buy | 22 bags |
| Per column/hole | 3.14 cu ft |
| Bags per column (no waste) | 5.2 bags |
| Total volume (with waste) | 13.19 cu ft |
Step by step:
- Radius = 12 in ÷ 24 = 0.5 ft
- One column = π × 0.5² × 4 ft = 3.142 cu ft
- 4 columns + 5% waste = 13.195 cu ft (0.489 yd³)
- Bags: 13.195 ÷ 0.6 = 21.99 → 22 bags of 80 lb
Bags of concrete per tube or post hole
One column, no waste added, 80 lb bags (0.60 cu ft each).
| Diameter × depth | Per column | Bags per column |
|---|---|---|
| 6 in × 3 ft | 0.59 cu ft | 1 bags |
| 8 in × 4 ft | 1.4 cu ft | 2.3 bags |
| 10 in × 4 ft | 2.18 cu ft | 3.6 bags |
| 12 in × 4 ft | 3.14 cu ft | 5.2 bags |
| 16 in × 4 ft | 5.59 cu ft | 9.3 bags |
| 24 in × 1 ft footing | 3.14 cu ft | 5.2 bags |
Common mistakes to avoid
- Using the diameter instead of the radius in π × r².
- Rounding up per hole instead of for the whole job — calculate all holes together and round once.
- Ignoring the bell or wider base some footings need.
Frequently asked questions
How many bags of concrete for a 12-inch Sonotube 4 ft tall?
A 12 in × 4 ft tube holds about 3.14 cu ft, which is about 5.2 bags of 80 lb mix — so buy 6 per tube, or calculate several tubes together to round up only once.
What diameter should a deck footing be?
It depends on the load, soil and local code. Many deck guides use 8–12 in tubes on footings, but your permit plans should set the size.