Sonotube Calculator — Concrete for Round Columns

Cardboard form tubes for deck footings and column pads: diameter and height in, bag count out.

Sonotube / Round Footing Calculator

What a sonotube is for

A 'Sonotube' (the brand that became the generic word) is a heavy cardboard cylinder set in the ground as a form, so a deck footing or column comes out round, smooth and plumb. Typical deck footings are 10–12 inch tubes sized to the beam load, poured down to the frost line with a footing pad or flared base at the bottom.

Size the tube to the load, not the hole

The tube diameter follows the load: a typical residential deck footing for one post is 10–12 inches; a ground-level porch post may only need 8. Your local deck code table (based on tributary area and soil bearing) is the authority — the tube should also be a couple inches wider than the post base hardware so anchors have room.

Pouring tips

Wet the tube inside so it does not steal water from the mix, pour in lifts and rod each lift to push out air, and set the anchor bolt while green. Leave the tube on through curing — stripping it too early in freeze country exposes green concrete.

FAQ

How much concrete is in a 12-inch sonotube?

A 12-inch tube holds about 0.79 cu ft per foot of height — so a 4-foot tube is roughly 3.15 cu ft, or about 5.3 of the 80-lb bags. The calculator does the exact math.

Do sonotubes need to be below the frost line?

Yes — attached decks in freeze climates require footings to the local frost depth, or seasonal heaving will lift and rack the structure.

Can I leave the cardboard tube in the ground?

Below grade, yes — it is standard practice. Above grade, most manufacturers say to remove or cut it back, because a wicking cardboard collar holds water against the column.