Estimate concrete for round sonotube forms. Enter the tube diameter, depth and number of piers to get cubic yards and bags instantly.
Volume per tube = π × radius² × depth. One 80 lb bag yields about 0.6 cubic feet.
A sonotube is a round cardboard form used to pour concrete piers for decks, fence posts and foundations. To figure the concrete, use the cylinder volume formula — pi times radius squared times depth — then convert to cubic yards by dividing by 27.
| Tube Diameter | Concrete per Foot of Depth |
|---|---|
| 6 inches | 0.20 cu ft |
| 8 inches | 0.35 cu ft |
| 10 inches | 0.55 cu ft |
| 12 inches | 0.79 cu ft |
| 14 inches | 1.07 cu ft |
| 16 inches | 1.40 cu ft |
For a deck with six 12-inch piers at 4 feet deep, you need about 6 × 4 × 0.79 = 18.9 cubic feet, or 0.7 cubic yards — roughly 32 bags of 80 lb mix.
The tube should be 2 to 4 inches wider than the post it supports, and it must extend below the frost line in your area. A typical deck uses 8 or 10 inch tubes for 4×4 posts and 12 inch tubes for 6×6 posts. Check local code for the required depth — footings that sit above the frost line heave and crack in winter.
Order about 10% extra concrete for every pour. Sonotubes are narrow, so a little spillage over the top or a low spot in the hole adds up fast when you have six or eight piers to fill.
A 12 inch sonotube uses about 0.79 cubic feet of concrete per foot of depth. A 4 foot deep tube needs about 3.16 cubic feet, or 6 bags of 80 lb mix per tube.
Use an 8 inch tube for a 4×4 post, leaving 2 inches of concrete around the post on every side. For a 6×6 post, use a 12 inch tube.
Below the frost line in your area, typically 3 to 4 feet in most of the US. Check local building code — footings above the frost line will heave and crack.
Yes, sonotubes are the standard way to pour deck footings. Set them in place, level them, and pour concrete to just above ground level.
For a deck or fence footing, a single vertical rebar in the center of each tube is recommended. For heavy loads or tall piers, add more per local code.