How Much Concrete for a Sonotube? Bags per Tube

How much concrete for a sonotube depends on the tube’s diameter and how deep it goes. A cardboard tube form is a cylinder, so the concrete volume is the circle’s area times the depth. A 12-inch tube holds about 0.79 cubic feet per foot of depth, which is roughly 1.3 bags of 80-pound mix per foot. An 8-inch tube holds less than half that, and an 18-inch tube more than twice as much.

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The formula

Volume in cubic feet = 3.14 × (diameter in feet ÷ 2)² × depth in feet.

For a 12-inch tube 4 feet deep: 3.14 × 0.5 × 0.5 × 4 = 3.14 cubic feet. At about 0.6 cubic feet per 80-pound bag, that is just over 5 bags.

Concrete per foot of tube

Tube diameter	Cubic feet per foot	80 lb bags per foot	60 lb bags per foot
6 inches	0.20	0.33	0.44
8 inches	0.35	0.58	0.78
10 inches	0.55	0.91	1.21
12 inches	0.79	1.31	1.75
14 inches	1.07	1.78	2.38
16 inches	1.40	2.33	3.10
18 inches	1.77	2.95	3.93
24 inches	3.14	5.24	6.98

Bag yields assume about 0.6 cubic feet per 80-pound bag and 0.45 per 60-pound bag. Round up to whole bags for each tube, and add one or two extra for the job.

Example: a deck with 8 footings

Eight 10-inch tubes, each 42 inches deep (3.5 feet):

Per tube: 0.55 × 3.5 = 1.93 cubic feet.
All tubes: 1.93 × 8 = 15.4 cubic feet.
In cubic yards: 15.4 ÷ 27 = 0.57.
In 80-pound bags: 15.4 ÷ 0.6 = about 26 bags, so buy 27 or 28.

That is a lot of mixing, but it can be done in batches since each tube is a separate pour. Our deck footing concrete guide covers footing sizes in more detail.

Account for the hole, not just the tube

The tube controls the shape above ground and near the top, but the bottom of the hole often has extra space:

Bell or spread footings. Many codes require a wider base below the tube, either a poured pad or a plastic bell form. Figure that volume separately.
Oversized holes. Augers and shovels often dig wider than the tube. If the tube sits on undisturbed soil and you backfill around it, extra concrete is not needed. If you fill the space around the tube with concrete, measure the hole.
Gravel at the bottom. Some designs call for a few inches of gravel under the footing for drainage; it reduces the concrete needed slightly.

Depth matters

Footings usually go below the local frost line so frost heave does not lift them. That depth ranges from a few inches in warm climates to 4 feet or more in cold ones. Check with your building department, which also sets the required diameter for decks and other structures.

Mixing tips

Mix only what you can place in a few minutes.
Rod or vibrate each tube as you fill it to remove air pockets.
Set anchors or post bases while the concrete is still workable, checking alignment with a string line.
Cut the tube off below grade or peel the exposed part once the concrete cures, if appearance matters.

Questions people ask

How many bags of concrete for a 12 inch sonotube?

About 1.3 bags of 80-pound mix per foot of depth. A 4-foot-deep tube takes just over 5 bags.

How many bags for an 8 inch sonotube 4 feet deep?

About 1.4 cubic feet, or roughly 2.3 bags of 80-pound mix, so plan on 3 bags.

How do you calculate concrete for a round column?

Multiply 3.14 by the radius in feet squared, then by the depth in feet. Divide by 27 for cubic yards.

Do I need extra concrete for a bell footing?

Yes. Figure the bell or spread footing volume separately and add it to the tube volume.

How deep should sonotube footings go?

Usually below the local frost line, which your building department can confirm, along with the required diameter.

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