Sonotube Sizing Guide
How to pick the right sonotube diameter and depth for deck footings, fence posts, and column piers.
Sonotubes are cardboard forming tubes that hold wet concrete in place while it cures, creating round concrete piers and footings. They are widely used for deck footings, porch supports, fence and mailbox posts, and pergolas. Choosing the correct diameter and depth is what makes the footing safe, so this guide walks through both.
Choosing a Diameter
- 8 inch: fence posts, mailbox posts, and light structures.
- 10–12 inch: the most common sizes for residential deck piers.
- 16 inch and larger: heavier porch or structural columns, or where soil is weak.
The right size depends on the load being carried and the strength of the soil. Deck footing sizes are often set by your local building code or a span table, so confirm the requirement before you buy tubes.
Choosing a Depth
Footings must reach below your local frost line so that freezing ground does not heave them upward. Frost depth ranges from about 12 inches in warm areas to 48 inches or more in cold northern regions. Your building department can tell you the exact figure for your address, and it also decides whether a permit and inspection are required.
How Much Concrete Does a Sonotube Need?
Tube volume is a cylinder: π × radius² × height. The table shows the volume of concrete per foot of tube depth and how many 80 lb bags (0.6 cu ft yield each) that means.
| Tube diameter | Concrete per foot of depth | 80 lb bags per foot |
|---|---|---|
| 8 in | 0.35 cu ft | 0.6 |
| 10 in | 0.55 cu ft | 0.9 |
| 12 in | 0.79 cu ft | 1.3 |
| 16 in | 1.40 cu ft | 2.3 |
| 18 in | 1.77 cu ft | 2.9 |
| 24 in | 3.14 cu ft | 5.2 |
Worked Example
A 12 inch tube set 4 feet deep needs 0.79 × 4 = about 3.1 cubic feet of concrete. That is about 5.2 bags of 80 lb mix, so buy 6. For a deck with six footings, multiply by six and round up, and consider ordering a little extra.
Installation Tips
- Dig to undisturbed soil at the required depth and keep the bottom flat.
- Set the tube plumb, brace it, and backfill around it before pouring.
- Keep the tube top slightly above grade, then cut it flush after the concrete has cured.
- Pour in lifts and work out air pockets with a rod.
- Set the anchor bolt or post bracket into the wet concrete, or use rebar as the design requires.
Mistakes to Avoid
- Stopping above the frost line to save digging.
- Choosing a diameter by guesswork rather than local code or span tables.
- Digging an oversize hole for a small tube, which wastes backfill effort and leaves the tube unsupported.
Use the sonotube calculator to enter your diameter and depth and see exactly how many bags or cubic yards you need.