What is a Sonotube?
A Sonotube (a brand name that has become generic) is a cylindrical cardboard form used to pour concrete columns and piers. They are commonly used for deck footings, fence post footings, sign posts, and foundation piers. The tube is placed in a hole, filled with concrete, and left in place as the concrete cures. The cardboard eventually deteriorates underground.
Accurately calculating the concrete volume needed for sonotubes prevents waste and ensures you purchase the right amount of concrete or premixed bags. Running short during a pour can compromise the structural integrity of the footing.
Sonotube Volume Formula
One 80-lb bag of premixed concrete yields approximately 0.6 cubic feet.
Common Sonotube Sizes
| Diameter | Volume per foot | Common Use |
|---|---|---|
| 8" | 0.35 cu ft | Fence posts, light-duty |
| 10" | 0.55 cu ft | Small deck piers |
| 12" | 0.79 cu ft | Standard deck footings |
| 16" | 1.40 cu ft | Heavy-duty piers |
| 24" | 3.14 cu ft | Large structural columns |
Worked Example
Four 12" diameter sonotubes, 48" deep, with 10% waste:
Total = 3.14 × 4 × 1.10 = 13.82 cu ft
80-lb bags = 13.82 / 0.6 = 24 bags
Frequently Asked Questions
How deep should sonotubes be?
Sonotubes must extend below the frost line in your area to prevent heaving. Check local building codes for frost depth requirements—typically 36–48 inches in northern climates and 12–24 inches in milder regions.
Should I use premixed bags or ready-mix truck?
For small projects (under 1 cubic yard), premixed bags are convenient and practical. For larger pours or many tubes, ordering from a ready-mix truck is more economical and ensures consistent concrete quality.
Do I need rebar in sonotubes?
For structural footings, most building codes require at least one vertical rebar (#4 or #5) in each tube, and sometimes a rebar cage. Check local codes and your structural engineer's specifications.