A concrete column or pier is a cylinder, and the volume formula is V = π × radius² × height. The calculator takes the column diameter in inches, converts it to feet (radius = diameter ÷ 24), and multiplies by the height in feet. The result is multiplied by the quantity for multiple identical columns. A 10% waste allowance covers spillage and over-fill at the top of the form.
A worked example: a 12-inch diameter pier 4 feet tall. Radius = 12/24 = 0.5 feet. Volume = π × 0.5² × 4 = 3.14 cubic feet. With 10% waste: 3.45 ft³ = 0.13 cubic yards. In 80 lb bags: 6 bags per pier. For four piers: 25 bags total — a manageable bagged-concrete job.
The column shape matters for rebar design but not for volume. A square column of the same width has a slightly larger cross-section than a round column of the same diameter (12-inch round vs 12-inch square: 113 in² vs 144 in², a 27% increase). For piers, round sonotubes are the standard because they're easy to set and brace. For visible columns (porch posts, pergola columns), square forms look more architectural.
Concrete per pier — common sonotube sizes (10% waste)
Diameter
Per 4′ pier (ft³)
Per 4′ pier (80 lb bags)
Per 8′ column (ft³)
Per 8′ column (80 lb bags)
8 inch
1.54
3
3.07
6
10 inch
2.4
4
4.8
8
12 inch
3.46
6
6.91
12
14 inch
4.7
8
9.41
16
16 inch
6.14
11
12.29
21
18 inch
7.78
13
15.55
26
12 inch
3.46
6
6.91
12
Choosing column diameter and depth
Column diameter depends on the load it carries and the soil's bearing capacity. For residential deck piers, 10–12 inches is standard for posts up to 8 feet tall. For porch columns or pergola posts, 12–16 inches gives more visual presence. For load-bearing columns (carrying roof or floor loads), the diameter is specified by an engineer — typically 16–24 inches for a 2-story residential column.
The soil-bearing capacity determines the column's footprint. A 12-inch pier on 2000 psf soil carries 1,570 pounds (π × 6² × 2000 / 144). On 4000 psf soil, the same pier carries 3,140 pounds. If the calculated load exceeds the soil capacity, you need a larger diameter or a pad footing at the base. Geotechnical testing ($500–1,500) gives you exact soil capacity; without testing, design to 2000 psf for clay, 3000 for sand, 4000 for gravel.
Depth must extend below the frost line in cold climates (36–48 inches in northern US) or to solid bearing in any climate. For non-frost areas, 24 inches minimum is typical for decks. The column must extend at least 6 inches above grade to keep wood posts away from soil moisture — a critical detail for deck post longevity. Use a post anchor (Simpson ABU or similar) embedded in the wet concrete to attach the post.
For sonotube piers, brace the tube securely before pouring. A sonotube full of wet concrete weighs 60–110 lb per foot of height (depending on diameter); a 12-inch tube 4 feet tall carries 250+ pounds of concrete and will topple if not braced in two directions. Use 2×4 lumber run from stakes driven into the ground to the top of the sonotube, screwed in place. Check plumb on two sides before and during the pour.
Rebar, sonotubes, and what the column estimate excludes
Most piers under 4 feet tall with light loads (deck posts, fence corners) don't need rebar — the concrete's compressive strength is more than enough for the load. For taller piers or heavier loads (porch columns, pergola posts), use 2–4 #4 vertical bars tied to a #2 horizontal hoop at the top. For structural columns carrying roof loads, an engineered rebar cage is required (4–6 #5 vertical bars with #3 hoops at 12-inch spacing).
Rebar must extend from the bottom of the pier (with 3-inch cover) up through the top, where it can be tied to the column above or bent into an L-shape to anchor the post base. The rebar also keeps the pier from shearing off at the ground line if the soil heaves. For deck piers, a single #4 bar vertical in the center is the minimum reinforcement; two or four verticals tied with hoops is better.
Sonotubes (cardboard concrete forms) are the standard for round piers. They come in 6, 8, 10, 12, 14, 16, and 18-inch diameters, in 4-foot and 8-foot lengths, and cost $15–50 per tube depending on size. Sonotubes are single-use; once the concrete cures, you peel off the cardboard (or leave it to biodegrade over a season). For reusable forms, steel or plastic column forms are available but cost $50–200 per form.
What the column calculator excludes: rebar ($0.50/lf), sonotubes ($15–50 each), post anchors or embedded hardware ($10–30 each), gravel base in the bottom of the hole (4 inches, $1–3 per pier), excavation ($25–100 per hole depending on soil and access), and finishing tools. For bagged concrete work, plan on 0.5 gallon of water per 80 lb bag — 6 gallons for a 12-bag pour, which is significant if you're working remote without a hose.
Cure time before loading: 7 days for deck posts, 14 days for columns carrying roof or floor loads, 28 days for full design strength. In cold weather, double these times. Use insulated blankets ($20–40 rental) if temperatures drop below 40°F during the first 3 days — concrete stops curing below freezing and can be permanently weakened.
Common mistakes and how to avoid them
The most common column mistake is undersized diameter for the load. A 10-inch pier on 2000 psf soil carries 1,090 pounds (pi times 5 squared times 2000 divided by 144); a 12-inch pier carries 1,570 pounds, and a 16-inch pier carries 2,790. If the calculated load on a deck pier exceeds the soil bearing capacity, the pier will settle — slowly at first, then progressively. When in doubt, go wider. The additional concrete is cheap; the cost of a settled deck is not.
A second mistake is inadequate depth. The column must extend below the frost line (36-48 inches in northern US) or to solid bearing, whichever is deeper. A pier set at 24 inches in a frost climate will heave 1-3 inches per winter as ice forms under the base, eventually failing the structure above. The cost of digging an extra 12-24 inches is small; the cost of a heaved deck post is large.
Sonotube bracing is the third common error. A sonotube full of wet concrete weighs 60-110 lb per foot of height. A 12-inch tube 4 feet tall carries 250+ pounds of concrete and will topple if not braced in two directions. Brace every tube with 2×4 lumber run from ground stakes to the top of the tube, screwed in place. Check plumb on two sides before and during the pour. A leaning sonotube produces a leaning pier, and a leaning pier is a structural problem.
Rebar placement in columns is frequently wrong. For piers under 4 feet tall with light loads, a single #4 bar vertical in the center is the minimum. For taller piers or heavier loads, use 2-4 #4 vertical bars tied to #2 horizontal hoops at 12-inch spacing. The rebar must extend from 3 inches above the bottom of the hole to 2 inches below the top of the concrete — rebar that stops short of the bottom provides no bending resistance. Tie the bars together with tie wire at every intersection; loose bars shift during the pour and provide no reinforcement.
Finally, do not load a fresh column too early. Concrete reaches 50% strength in 3-7 days and full design strength in 28 days. Loading a column at 24 hours (common on rushed deck jobs) can permanently weaken it. Wait at least 3 days for deck posts, 7 days for porch columns, and 14 days for structural columns carrying roof or floor loads. In cold weather (below 50 degrees F), double these times — concrete curing slows dramatically as temperature drops.
FAQ
Built and maintained by Rizwan. Calculations are performed client-side using published manufacturer bag yields, ACI mix ratios, and ASTM rebar specifications. Every formula and worked example on this site can be verified by hand against the sources listed below.
Sources: Quikrete technical data sheets; ACI 318 Building Code Requirements for Structural Concrete; ASTM A615 rebar specifications; US Concrete Institute material weights.
Last reviewed: August 2026. Prices reviewed quarterly. Affiliate disclosure: this page may contain Amazon affiliate links. See privacy policy.