How Much Concrete Do You Need for a Sonotube?

Updated 2026-07-19

Sonotubes — the cardboard forming tubes stacked by the lumber aisle — are how you pour clean, round footings for decks, pergolas, and pole barns without building a single form. The concrete math is pure cylinder volume, and once you know three numbers, sizing a whole deck of footings takes about two minutes.

The three numbers that matter are the tube diameter, the total depth (below grade plus whatever sticks up as a pier), and the yield of the bags you are buying. Everything else is multiplication.

Volume per foot of tube height

Those come straight from pi r squared h. An 8-inch tube has a 4-inch radius, so each 12 inches of height holds about 603 cubic inches — 0.35 cubic feet. Translated to bags: every foot of an 8-inch tube takes a little over half an 80 lb bag, and every foot of a 12-inch tube takes about 1.3 bags.

1. 8-inch tube: 0.35 cu ft per foot of height.
2. 10-inch tube: 0.55 cu ft per foot.
3. 12-inch tube: 0.79 cu ft per foot.

Worked example: six deck footings

Say your deck plan calls for six footings in 10-inch tubes, poured 4 feet deep to get below the frost line. Each footing takes 0.55 × 4 = 2.2 cubic feet, and six of them need 13.2 cubic feet — just under half a cubic yard. In bags, that is 13.2 ÷ 0.60 = 22 bags of 80 lb mix, or 30 bags of 60 lb. Toss in one or two spares for the hole that sloughs or the tube that sits crooked.

If the tubes extend above grade to form piers, include that height in the length. A footing 42 inches deep with 8 inches of pier showing is 50 inches of tube — 4.2 feet of volume, not 3.5.

Setting a post in the tube? Subtract its volume

If you bury a wood post directly in the footing, the post displaces concrete. A 4x4 (3.5 inches actual) set 3 feet into the tube takes up about a quarter of a cubic foot — less than half a bag per footing, but across a dozen footings it is real money.

Most modern decks skip the buried post entirely: pour the tube full, set a galvanized post base in the wet concrete, and bolt the post on later. It keeps wood out of the ground, passes inspection more easily, and makes the math simpler — full tube volume, no subtraction.

Depth is about frost, not preference

Footing depth is set by your local frost line — anywhere from 12 inches in the deep South to 48 inches or more in the upper Midwest and New England. Pour shallower than frost depth and freeze-thaw cycles will jack the footing upward, taking the deck with it. The building department publishes the number, and the inspector will want to see the holes before you pour.

Diameter is about load and soil. Eight-inch tubes suit light pergolas and landings; 10- and 12-inch tubes are the norm for deck footings. Weak soil or a heavy roof load may push you wider — that is a span-table question for your plans examiner, not a guess.

Field tips that save bags and headaches

Once you have your tube diameter and total depth, drop them into the concrete calculator for an exact bag count across the whole footing schedule — it beats doing cylinder math in the lumber aisle.

1. Dig the hole an inch or two wider than the tube so you can plumb it, then backfill around the outside before pouring — an unbraced tube bows, and a bowed tube wastes concrete.
2. Extend the tube 2 to 3 inches above finished grade so post bases stay clear of standing water and mulch.
3. Pour in lifts and rod each one with a stick or rebar to chase out air pockets, especially below grade where you cannot see voids.
4. Order about 10% extra — cardboard tubes are never perfectly round once they are in the ground.