Concrete Tube Calculator

Eight-inch tubes suit light loads like a small landing or pergola post; 10 and 12 inch are the usual choice under a residential deck post. Size follows the load: the IRC counts 40 psf live plus 10 psf dead, so a post carrying 60 square feet of deck picks up about 3,000 pounds. On 1,500 psf soil that needs 2 square feet of bearing — roughly a 20-inch circle, which is why many decks use a wider pad or flared base under a 12-inch column. This is a planning estimate; footing size is set by local code and your inspector.

Deep enough that the bottom bears below the local frost line, and never less than 12 inches below undisturbed ground. Frost depth is effectively zero in south Florida, 18–24 inches across much of the mid-Atlantic, 36 inches in the northern plains, and 42–48 inches in northern Minnesota and New England. Your building department publishes the number for your address — ask before you dig, because a pier that stops short of frost depth will heave and lift the structure over a winter or two. Depth also has to reach undisturbed soil, not backfill.

A flared footing spreads the load over more soil, which is how a 12-inch column can land on a 20-inch bearing surface. This calculator measures the straight column only, so add the base volume separately. A 20-inch-diameter pad 8 inches thick is about 1.45 cubic feet, or roughly 2.5 additional 80 lb bags per pier. Some regions require the bell instead of allowing a plain column; others prohibit bells in expansive clay because frost can grab the flare. Check the local requirement before you form anything.

One cubic yard is 45 80-lb bags. Six 12-inch tubes poured 4 feet deep come to about 0.70 cubic yards — roughly 32 bags, which a rented mixer handles in an afternoon. Past about one cubic yard, or 45 bags, ready-mix usually wins on both cost and consistency once you factor in the short-load fee that most plants charge on orders under 3–4 yards. Bags also let you pour piers one at a time, which matters when holes are scattered around a sloped yard and a truck chute will not reach.

The buried portion stays — it is a form, not a structural element, and it rots away harmlessly. Strip and discard whatever sticks up above grade once the concrete has set, usually the next day, because a wet cardboard collar traps moisture against the pier and looks rough. Score the exposed section with a utility knife and peel it. Set anchor bolts or a galvanized post base into the wet concrete while you pour; posts should never sit directly on soil or in a pocket of the pier.