Calculate cubic yards, bags, and cost for round concrete columns, piers, and post holes
Round concrete columns are used for deck posts, fence posts, porch piers, and structural supports. The volume formula is different from rectangular shapes — here is everything you need to know.
Volume = pi x radius squared x height. In practical terms: measure the diameter in inches, divide by 24 to get the radius in feet, square it, multiply by pi (3.1416), then multiply by the height in feet. Divide by 27 for cubic yards.
Example: 12-inch diameter column, 4 feet tall = 3.1416 x (0.5)^2 x 4 = 3.14 cubic feet = 0.116 cubic yards.
| Diameter | Height | Cubic Feet | 80 lb Bags | Cubic Yards |
|---|---|---|---|---|
| 8 in | 2 ft | 0.70 | 2 bags | 0.026 |
| 10 in | 3 ft | 1.64 | 3 bags | 0.061 |
| 12 in | 3 ft | 2.36 | 4 bags | 0.087 |
| 12 in | 4 ft | 3.14 | 6 bags | 0.116 |
| 16 in | 4 ft | 5.59 | 10 bags | 0.207 |
| 18 in | 5 ft | 8.84 | 15 bags | 0.327 |
| 24 in | 8 ft | 25.13 | 42 bags | 0.931 |
Sonotube (cardboard concrete forms) come in standard sizes: 6, 8, 10, 12, 14, 16, 18, 20, 24, 30, and 36 inches in diameter. They are available at lumber yards and big-box stores. For deck footings, 10-12 inch diameter is most common. Cut to length with a handsaw.
For fence posts and deck posts in dry climates, the dry-pack method works well: set the post, pour dry concrete mix directly into the hole, then add water on top and let moisture from the soil do the work. This method requires no mixing and sets in 4 hours. Use Quikrete Fast-Setting or equivalent — do not use this method with standard concrete mix.
Use the formula: Volume = pi x (diameter / 2) squared x height. Convert diameter from inches to feet first (divide by 12). Example: a 12-inch diameter column 4 feet tall = 3.1416 x 0.25 x 4 = 3.14 cubic feet = 0.116 cubic yards. Our calculator does this automatically.
A 10-inch hole 2.5 feet deep needs about 1.5 bags of 80 lb concrete. A 12-inch hole 3 feet deep needs about 2-2.5 bags. A 12-inch hole 4 feet deep needs about 3 bags. A 16-inch hole 4 feet deep needs about 6 bags. Enter your dimensions in the calculator above for exact counts.
For a residential deck supporting standard loads, a 10-12 inch Sonotube is typical. The footing must extend below the frost line and be sized to bear the load — consult local building codes. Most residential deck pier footings are 10-12 inches diameter and 36-48 inches deep.
Structural concrete columns require vertical rebar (typically 4-6 bars) with horizontal ties every 12-16 inches. Deck pier footings may only require a single J-bolt or post anchor set in the wet concrete. Check your local building department — most jurisdictions require footing inspection before pouring.