Key takeaways
- Column volume = 3.14 × radius² × height, with everything in feet.
- Doubling the diameter takes four times the concrete.
- This tool gives volume, not the column size, depth or reinforcement you need.
Enter the inside diameter of the form, its height, and how many you're filling. This works for any round form, including tube forms sold under the Sonotube brand name.
How the calculator works
- Radius in feet: diameter in inches ÷ 2 ÷ 12
- Cubic feet: 3.14 × radius² × height × number of columns
- Cubic yards: cubic feet ÷ 27
- Bags: cubic feet ÷ yield per bag, rounded up
Worked example
Four columns, each 12 inches across and 4 feet high, with a 5% allowance:
- Radius: 12 ÷ 2 ÷ 12 = 0.5 feet
- One column: 3.14 × 0.25 × 4 = 3.14 cubic feet
- Four columns: 12.6 cubic feet, or 13.2 with the allowance
- About 0.49 cubic yards, or 22 bags of 80 lb, 30 of 60 lb, or 44 of 40 lb
Concrete per foot of height
These are before any allowance. Bags are shown as exact figures, so round up your total.
| Diameter | Cubic feet per foot | 80 lb bags per foot |
|---|---|---|
| 8 in | 0.35 | 0.58 |
| 10 in | 0.55 | 0.91 |
| 12 in | 0.79 | 1.31 |
| 16 in | 1.40 | 2.33 |
| 18 in | 1.77 | 2.95 |
| 24 in | 3.14 | 5.24 |
Before you pour
- Size and depth: the 2021 International Residential Code's frost rules cover piers and other permanent supports of buildings, and deck footings have their own section, R507.3. Ask your building department what applies and whether you need a permit.
- Reinforcement: whether a column needs steel depends on its design. Ask your building department or an engineer.
- Buried utilities: contact 811 a few business days before you dig.
How much diameter matters
Going up one common size uses far more concrete than it looks like:
| From | To | Extra concrete per foot |
|---|---|---|
| 8 in | 10 in | +56% |
| 10 in | 12 in | +44% |
| 12 in | 16 in | +78% |
So check your required diameter before you buy forms or concrete.
Frequently asked questions
How many bags of concrete does a 12-inch Sonotube take?
About 1.31 bags of 80 lb for each foot of height, based on QUIKRETE's 0.60 cubic foot yield. A 4-foot column takes about 5.2 bags, so buy 6.
Does the part of the form below ground count?
Yes. Use the full height of concrete, including any part below ground and any bell or footing added separately.
Why does a small increase in diameter use so much more concrete?
Volume rises with the square of the radius. An 8-inch form takes 0.35 cubic feet per foot, and a 16-inch form takes 1.40, which is four times as much.
Sources
We use primary sources where we can: building codes, federal rules and manufacturer documents. Read our editorial standards.
- 2021 International Residential Code, Chapter 4, Section R403.1.4.1 (frost protection)
- 811: What is 811?
- QUIKRETE: Concrete Mix data sheet, product 1101 (bag yields)