The volume is just the area of the circle times the height
A round column is a cylinder, so the concrete it holds is the circle area (π times the radius squared) times the height. Work in feet and you get cubic feet; divide by 27 to reach the cubic yards concrete is sold in. The diameter is the outside of the form (Sonotube), so use half of it as the radius.
Concrete per column, by diameter and height
| Diameter | Per foot of height | 8 ft column | 10 ft column |
|---|---|---|---|
| 8 in | 0.35 ft³ | 2.8 ft³ (0.10 yd³) | 3.5 ft³ (0.13 yd³) |
| 10 in | 0.55 ft³ | 4.4 ft³ (0.16 yd³) | 5.5 ft³ (0.20 yd³) |
| 12 in | 0.79 ft³ | 6.3 ft³ (0.23 yd³) | 7.9 ft³ (0.29 yd³) |
| 16 in | 1.40 ft³ | 11.2 ft³ (0.41 yd³) | 14.0 ft³ (0.52 yd³) |
| 18 in | 1.77 ft³ | 14.1 ft³ (0.52 yd³) | 17.7 ft³ (0.65 yd³) |
Doubling the diameter quadruples the concrete, because the radius is squared: a 16 in column takes four times as much as an 8 in one of the same height.
Bags or ready-mix
Both fill the same volume; the choice is labor. An 80 lb bag of mix yields about 0.6 cubic feet after water, so bag counts climb fast as columns get wider.
Yield per bag after mixing. A 12 in by 8 ft column (6.3 ft³) needs about 11 eighty-pound bags — near the point where a ready-mix load is easier and avoids weak seams between batches.
- Add 5 to 10% waste. Spillage, an over-dug hole and a slightly deep footing all eat concrete. Order a little over the exact figure.
- One pour, not layers. Filling a tall form in separate batches hours apart creates a cold joint — a weak line where the column can crack. Mix enough to pour it in one go.
Common questions
How much concrete does a round column need?
Multiply pi by the radius squared by the height, all in feet, to get cubic feet, then divide by 27 for cubic yards. A 12 inch diameter column 8 feet tall holds about 6.3 cubic feet, or 0.23 cubic yards.
How many bags of concrete for a column?
An 80 lb bag yields about 0.6 cubic feet, a 60 lb bag about 0.45, and a 40 lb bag about 0.3. Divide the columns cubic feet by the bag yield. The 12 inch by 8 foot column above takes roughly 11 of the 80 lb bags.
Where the bag vs ready-mix line usually falls
Bags are practical up to roughly 0.5 cubic yards, or a dozen 80 lb bags. Past that the mixing labor and the risk of cold joints between batches make ready-mix delivery cheaper and stronger, and most suppliers set a 1 cubic yard minimum.
