Quick answer: V = π × (diameter ÷ 2)² × depth, then multiply by the number of piers. A 12-inch tube at 4 ft deep takes 3.14 ft³ ≈ 0.12 yards ≈ 6 × 80 lb bags. Ten of those piers is 31.4 ft³ ≈ 1.16 yards — 53 bags if you mix continuously, or 60 if you buy six whole bags per tube.
Sonotube size chart
Concrete per foot of depth, and 80 lb bags for the two most common pier depths:
| Tube ⌀ | ft³ per ft | 3 ft deep | 4 ft deep | 80 lb bags (4 ft) |
|---|---|---|---|---|
| 6 in | 0.196 | 0.59 ft³ | 0.79 ft³ | 2 |
| 8 in | 0.349 | 1.05 ft³ | 1.40 ft³ | 3 |
| 10 in | 0.545 | 1.64 ft³ | 2.18 ft³ | 4 |
| 12 in | 0.785 | 2.36 ft³ | 3.14 ft³ | 6 |
| 14 in | 1.069 | 3.21 ft³ | 4.28 ft³ | 8 |
| 16 in | 1.396 | 4.19 ft³ | 5.59 ft³ | 10 |
| 18 in | 1.767 | 5.30 ft³ | 7.07 ft³ | 12 |
| 24 in | 3.142 | 9.42 ft³ | 12.57 ft³ | 21 |
Because the diameter is squared, stepping up one size costs far more concrete than it looks. Going from a 10-inch to a 12-inch tube adds only two inches of width but 44% more concrete per foot.
Depth, frost and footing pads
Pier depth is not a design choice — it is set by your local frost line. Ground that freezes expands, and a footing sitting above that depth gets lifted every winter, taking the deck or shed with it. Frost depths range from nothing in warm climates to four feet or more in northern regions, and your building department publishes the figure that applies to you.
Many jurisdictions also require a wider footing pad at the base of the tube — a bell or a square pad that spreads the load over more soil. If yours does, calculate the pad separately as a cylinder or a slab and add it to the tube volume. A 24-inch pad 8 inches thick adds about 2.1 ft³ per pier, which is most of another two bags.
This calculator sizes the pour. It does not size the structure: pier diameter, spacing and pad dimensions depend on the load and the soil's bearing capacity, and those belong to your local code or an engineer.
Common mistakes & pro tips
- Diameter entered as radius. A 4× error. Tubes are always sold by diameter.
- Forgetting the footing pad. If code requires one, it can add half again to the concrete per pier.
- Measuring the hole instead of the tube. Concrete fills the tube, not the over-dug hole around it. Backfill the gap with soil or gravel.
- Pouring above the frost line. The single most common cause of heaving decks.
- Pro tip — brace before you pour. Wet concrete will float and tilt an unbraced tube. Stake and level every tube first, and check them again after the first tube is filled.
- Pro tip — two bag numbers, both useful. The calculator shows the total (mixing continuously across all tubes) and the per-column count. Buying whole bags per tube always costs a few more, but it is easier to work with when you pour piers one at a time.
- Pro tip — count bags honestly. Ten 12-inch piers is over 50 bags — well past a ton to carry and mix. Beyond about 25 bags, price a short-load ready-mix delivery.
Frequently asked questions
How much concrete does a sonotube hold?
An 8-inch tube holds about 0.35 cubic feet per foot of depth, a 10-inch 0.55, and a 12-inch 0.79. So a 12-inch tube poured 4 feet deep takes 3.14 cubic feet — about 0.12 cubic yards, or roughly five 80 lb bags.
How many bags of concrete per sonotube?
For a 4-foot depth: an 8-inch tube takes about 3 bags of 80 lb, a 10-inch takes 4, and a 12-inch takes 6. A ten-pier deck on 12-inch tubes needs 31.4 cubic feet in total, which is 53 bags mixed continuously — or 60 if you work one tube at a time from whole bags. Either way, that is the point where ready-mix starts to make sense.
How deep should deck footings be?
Below the local frost line, which ranges from zero in warm regions to 48 inches or more in cold ones. Footings that sit above the frost line heave when the ground freezes and lift the structure with them. Your building department publishes the required depth for your area.
What diameter sonotube do I need?
Deck piers commonly use 8 to 12 inch tubes, with the size driven by the load each post carries and the soil's bearing capacity. Many jurisdictions also require a wider footing pad at the base of the tube to spread the load. Check your local code rather than matching what a neighbour used.
Should I leave the sonotube in the ground?
The buried portion can stay — it degrades harmlessly over time. Cut the tube off flush or slightly below grade at the top, because cardboard left above ground holds moisture against the concrete and looks untidy as it weathers.