Calculate concrete volume for round and square columns used in decks, porches, and carports.
Concrete columns come in three main shapes, each suited to different applications. Round columns are the most popular choice for residential construction. They are formed using cardboard sonotube forms that come in standard diameters ranging from 6 inches to 24 inches for residential use, and up to 60 inches for commercial projects. Round columns distribute loads evenly across their cross-section, making them structurally efficient. They are the go-to choice for deck piers, porch supports, and fence posts set in concrete.
Square columns offer a clean, architectural look that works well with traditional building styles. They are formed using plywood or lumber, which makes the formwork more labor-intensive than sonotubes but allows for custom sizing. Square columns are common in porch construction, pergolas, and commercial buildings where a specific aesthetic is desired. Standard residential sizes range from 8 inches by 8 inches up to 16 inches by 16 inches.
Rectangular columns are used when space constraints require a column that is wider in one direction than the other. You will see these in basement construction where a column needs to fit against a wall, or in commercial structures where beam spans dictate an elongated column shape. They require more careful reinforcement design because the load distribution is not uniform across the cross-section.
For most deck and porch projects, round columns in sonotube forms are the easiest and most cost-effective choice. An 8-inch sonotube works for light-duty applications like small decks and shed foundations. A 12-inch tube is the standard for most residential decks and porches. Go with 16 inches or larger for heavy loads, multi-story structures, or when your soil bearing capacity is low. Square columns make sense when appearance matters, such as a front porch where the columns are visible, or when you are matching existing architectural elements on the house.
Column size depends on three factors: the load it carries, its height (unsupported length), and local building code requirements. As a general rule, a column's diameter should be at least one-twelfth of its unsupported height. A 10-foot tall column should be at least 10 inches in diameter. Most residential building codes specify minimum column sizes for specific applications. Your local building department can tell you exactly what is required for your project.
Building codes typically require vertical rebar in concrete columns. A common residential setup for a 12-inch round column uses four pieces of number 4 rebar spaced evenly around the inside of the form, held together with number 3 rebar ties every 12 inches. The rebar cage should have at least 1.5 inches of concrete cover on all sides. For square columns, the rebar is placed at each corner. Always check your local code, as requirements vary by region and application.
12-inch diameter round column, 10 feet tall:
Radius = 6 inches = 0.5 feet
Volume = pi x (0.5)^2 x 10 = 7.85 cubic feet
Convert: 7.85 / 27 = 0.29 cubic yards
With 10% waste: 0.32 cubic yards
That is approximately 12 bags of 80-pound concrete mix per column.
Most residential decks use 8-inch or 12-inch diameter round columns formed with sonotubes. For single-story decks under 10 feet tall, 8-inch columns are usually sufficient. For taller or multi-level decks, 12-inch columns provide more load capacity. Always check your local building code or consult an engineer for structural requirements specific to your project.
Yes, most building codes require vertical rebar in concrete columns. A common residential setup uses 2 to 4 pieces of number 4 rebar running vertically through the column, tied together with horizontal ties every 12 to 18 inches. Rebar dramatically increases the column's ability to resist lateral forces and prevents cracking under load.
Sonotubes are cardboard tube forms used to pour round concrete columns. They come in diameters from 6 inches to 60 inches. To use them, dig a hole to your footing depth, place the tube in the hole, brace it level and plumb, then pour concrete inside. After the concrete cures for 24 to 48 hours, you can peel the cardboard away or leave it in place underground.
A 12-inch diameter column that is 10 feet tall requires approximately 7.85 cubic feet or 0.29 cubic yards of concrete. That works out to about 12 standard 80-pound bags of concrete mix. Always add 10 percent extra for waste and spillage to avoid coming up short during the pour.
For most residential columns supporting decks, porches, and carports, 3000 PSI concrete is the standard. If your columns will bear heavier structural loads or are in areas with freeze-thaw cycles, consider 4000 PSI concrete. Commercial and industrial columns may require 5000 PSI or higher. Check with your local building department for specific requirements.