Your footings
Frost depth matters: your tube must reach below the local frost line (typically 30–48″ in northern states, 12–18″ in the South).
Your materials
How much concrete does a sonotube need?
A sonotube (cardboard form tube) needs concrete equal to the cylinder’s volume: π × radius² × depth. In practical terms, a common 10-inch tube takes about one 80 lb bag per foot of depth — so a 4-foot deck footing needs roughly 4 bags, and a 6-footing deck project needs about 24–27 bags including the standard 10% extra. At July 2026 prices ($5.40 per 80 lb bag), that’s around $145 in concrete plus $12–18 per tube form.
Bags per sonotube — quick reference
| Tube diameter | 80 lb bags per ft of depth | Bags for 4 ft footing | Cu ft per ft |
|---|---|---|---|
| 8″ | 0.6 | 3 | 0.35 |
| 10″ | 0.9 | 4 | 0.55 |
| 12″ | 1.3 | 6 | 0.79 |
| 14″ | 1.8 | 8 | 1.07 |
| 16″ | 2.3 | 10 | 1.40 |
| 18″ | 3.0 | 12 | 1.77 |
Sizing a footing correctly
Choosing tube diameter
Diameter is set by the load, not preference. Common practice: 8″ tubes for fence posts and light pergolas, 10–12″ for typical deck posts (the most common residential size), and 14–18″ for tall decks, hot-tub-bearing corners, and covered porch columns. When in doubt, size up — the extra bag or two of concrete costs less than $12, and an undersized footing can’t be fixed without digging it out. Local code may specify diameter based on your deck’s tributary load; check before you dig, and call 811 to locate utilities first.
Mixing tips for tube pours
Tube footings are the most beginner-friendly concrete job there is. Three things make them go well: mix slightly wet — a stiffer mix leaves voids in narrow tubes; consolidate by rodding with rebar or a stick every foot of pour to release trapped air; and brace the tube plumb before pouring, because a leaning footing is permanent. Set your post anchor into the wet concrete within 15–20 minutes of topping off, and let footings cure 48–72 hours before loading them.