Sonotube footings are cylinders. This calculator handles single tubes, batch totals (e.g. 6 tubes for a deck in one calculation), and includes 10% waste for over-fill at the top.
How Sonotube concrete is calculated
total = volume × tube count × 1.10
Standard fill convention: pour to 1-2 inches above the tube top, then trowel level. The 10% waste covers this overfill plus any spillage.
Standard Sonotube sizes
| Diameter | Use | Concrete per ft of height |
|---|---|---|
| 6" | Light decorative posts | 0.20 cu ft |
| 8" | Pergola posts, fence posts | 0.35 cu ft |
| 10" | Standard deck footings | 0.55 cu ft |
| 12" | Heavy deck, garage post | 0.79 cu ft |
| 16" | Structural columns, heavy deck | 1.40 cu ft |
| 18" | Carport, large structural | 1.77 cu ft |
| 24" | Commercial, very heavy | 3.14 cu ft |
10" and 12" are by far the most common for residential decks. Anything over 16" is engineered structural — get a structural engineer’s input.
Footing depth — frost line + structural load
1. Below frost line for your region (24-60" depending on climate)
2. Sufficient bearing area at the bottom for the load above
For most residential decks, a belled footing at the bottom of the Sonotube (a wider concrete pad) provides additional bearing. The belled section can be 16-24" diameter for a 12" Sonotube.
Reinforcement
For deck and load-bearing footings, install rebar before pouring:
- Vertical bars: 4 × #4 rebar (1/2" diameter), full height of the tube, set 1.5" from the cardboard wall
- Horizontal ties: #3 stirrups every 12" linking the verticals
- Anchor / J-bolt: Cast a J-bolt or Simpson post anchor in the top, centered, before the concrete sets
The rebar should sit on small dobies (concrete spacers) at the bottom so it’s not in direct contact with the dirt.
Concrete amount for typical projects
| Project | Tubes | Size | Total cu ft | Bags (80 lb) |
|---|---|---|---|---|
| Small deck (8×10) | 4 | 10" × 4 ft | 8.6 cu ft | 14 bags |
| Standard deck (12×16) | 6 | 12" × 4 ft | 18.4 cu ft | 31 bags |
| Large deck (16×24) | 9 | 12" × 4 ft | 27.7 cu ft | ~1 cu yd ready-mix |
| Pergola (12×12) | 4 | 8" × 6 ft | 6.0 cu ft | 10 bags |
| Mailbox post | 1 | 8" × 30" | 0.95 cu ft | 2 bags |
Above ~25 cu ft (close to 1 cu yd), ready-mix delivery becomes cheaper than bags.
Pouring technique
- Set the tube plumb in the hole (level on two sides)
- Backfill around the outside, tamping in lifts to hold it plumb
- Tie rebar cage and lower into the tube
- Add the J-bolt or post anchor with a positioning template
- Pour concrete in lifts, tamping or vibrating each lift to remove air pockets
- Slightly overfill, then trowel flush at the tube top
- Embed the J-bolt while concrete is still wet, leveling with a torpedo level
- Cover with plastic for 24 hours to slow water loss
Common mistakes
- Tube not plumb — post sits crooked, fixable only by removing and re-pouring
- No rebar — concrete cracks under lateral load (deck rocking, wind)
- No frost-depth bearing — footing heaves in winter, deck shifts
- Overfilled tube without troweling — top is uneven, post anchor not flush
Frequently asked questions
What size Sonotube for a deck?
10-12" diameter for residential deck footings supporting standard 4×4 or 6×6 posts. 16-18" diameter for heavy decks, hot tubs, or any structure over 12 ft above grade. Always check IRC R403 footing tables for your specific load.
How tall should a Sonotube extend above grade?
6-12" above grade for deck and pergola footings — keeps the post off wet ground. For decks with adjustable post bases, just enough above grade to keep the post hardware dry. Below grade: extends to your local frost line depth.
Do I need rebar in a Sonotube?
For non-structural posts: Optional. For deck and structural footings: Yes — typically 4 vertical #4 rebars with horizontal ties every 12". The rebar resists lateral loads and crack propagation. Tie a J-bolt or post anchor to the rebar before pouring.
Can I leave the Sonotube tube in place?
Yes — Sonotubes are designed to stay in place after pouring. The cardboard absorbs moisture and decomposes within 1-2 years underground. Above grade, you can peel the cardboard off after 7 days for a clean concrete look, or leave it (it'll weather away).
How many Sonotubes for a 12×16 deck?
Typical layout: 6 footings (4 corners + 2 mid-span). At 12" × 4 ft tubes: ~5.7 cu ft total = 0.21 cu yd = ~10 × 80 lb concrete bags. The calculator handles batch totals — enter 6 in the count field.