Deck piers, porch columns, fence posts and pole barn footings are all cylinders of concrete, poured into round cardboard tube forms or straight into augered holes. Their volume is π × radius² × height. A 12-inch tube 4 feet deep holds about 3.14 ft³, which is roughly five 80 lb bags. This concrete column calculator works out the volume for any number of round or square columns, with an optional wider bell or footing at the base. It gives the total in cubic yards and in bags, using manufacturers' published bag yields.
For flat work, use the concrete slab calculator. For deck framing and the number of piers you'll need, the deck board calculator and the American Wood Council's DCA 6 guide are good starting points.
How to use the concrete column & post hole calculator
- Choose round or square.
- Enter the inside diameter of the tube form or hole (for example 8, 10 or 12 inches), and its depth.
- Enter how many identical columns or holes you are pouring.
- If the base flares out (a bell footing or a square footing pad under the tube), enter its diameter and thickness.
- Keep 10% or more extra for post holes, which are rarely perfectly straight.
Formula
Bag yields used: 80 lb ≈ 0.60 ft³, 60 lb ≈ 0.45 ft³, 50 lb ≈ 0.375 ft³, 40 lb ≈ 0.30 ft³ (Quikrete's published yields for standard concrete mix). A bell footing is approximated as a short cylinder.
Worked example
Six deck piers in 12-inch tubes, 4 ft deep, 10% extra.
- One tube: π × 0.5² × 4 = 3.142 ft³.
- Six tubes: 18.85 ft³, or 20.73 ft³ with 10% (0.768 yd³).
- 80 lb bags: 20.73 ÷ 0.6 = 34.6, so 35 bags. In 60 lb bags: 47.
For a fence, a 10-inch hole 2.5 ft deep needs π × 0.4167² × 2.5 = 1.36 ft³, about 3 × 80 lb bags with 10% extra. Subtract the volume of the post if you want to be exact: a 4×4 post (3.5 in square) displaces about 0.21 ft³ over 2.5 ft.
Concrete per foot of tube
| Tube diameter | ft³ per foot of depth | 80 lb bags per foot |
|---|---|---|
| 6 in | 0.196 | 0.33 |
| 8 in | 0.349 | 0.58 |
| 10 in | 0.545 | 0.91 |
| 12 in | 0.785 | 1.31 |
| 14 in | 1.069 | 1.78 |
| 16 in | 1.396 | 2.33 |
| 18 in | 1.767 | 2.95 |
| 24 in | 3.142 | 5.24 |
Depth, frost and footing size
Piers and posts usually have to reach below the local frost depth so that frozen soil can't heave them, and they bear on undisturbed soil. Your building department sets the minimum depth. Footing diameter depends on the load and the soil's bearing capacity. DCA 6 gives prescriptive footing sizes for typical residential decks. If you pour more than about a cubic yard, compare the price of ready-mix delivery with bags; the cubic yard calculator converts volumes. For reinforcement in tall columns, follow your plans and use the rebar calculator for quantities.
Caveats and assumptions
- Bag yields are approximate and vary by product; check the bag label.
- Holes dug by hand or auger are rarely uniform, so keep the extra allowance.
- Bell footings are approximated as a cylinder at the base; tapered bells hold somewhat less.
- Footing depth and size must meet your local code and engineering requirements.
Frequently asked questions
How many bags of concrete for a 12-inch Sonotube 4 feet deep?
It holds π × 0.5² × 4 = 3.14 ft³. At about 0.6 ft³ per 80 lb bag, that is about 5.2, so 6 bags with a little spare.
How much concrete do I need for a fence post?
For a 10-inch hole 2.5 ft deep, about 1.36 ft³, or 3 × 80 lb bags with 10% extra. Wider or deeper holes need more.
How do I calculate the volume of a round column?
Multiply π by the radius squared by the height, in feet. Divide cubic feet by 27 for cubic yards.
How many 80 lb bags make a yard of concrete?
About 45, because a cubic yard is 27 ft³ and each 80 lb bag yields about 0.6 ft³.
Should I subtract the post from the hole volume?
You can, but most people don’t. A 4×4 post only displaces about 0.085 ft³ per foot, and the extra covers spillage.
Sources
- Quikrete — Concrete Calculator and product yields — yield per 80, 60, 50 and 40 lb bag
- American Wood Council — DCA 6 Prescriptive Residential Wood Deck Construction Guide — footing sizes and depth for deck posts
- Portland Cement Association — concrete construction guidance
This calculator gives estimates for planning and education. Check measurements, local codes, supplier specs and your school's official policy before relying on a result. See our disclaimer and how we build and check calculators.