How this calculator works
A concrete form tube (often sold as a Sonotube) is a cylinder, so the volume is π × (diameter ÷ 2)² × height. We work in inches, divide by 1,728 to convert cubic inches into cubic feet, add your waste allowance, and divide by the volume a bag yields: 0.60 ft³ for an 80 lb bag, 0.45 for 60 lb and 0.375 for 50 lb.
Worked example: a 12 inch tube filled 4 feet (48 inches) deep holds π × 6² × 48 ÷ 1,728 = 3.14 ft³. With 10% extra that is 3.46 ft³, or 6 bags of 80 lb.
Concrete per tube (cubic feet, no waste)
| Diameter | 2 ft | 3 ft | 4 ft | 5 ft | 6 ft |
|---|---|---|---|---|---|
| 6 in | 0.39 | 0.59 | 0.79 | 0.98 | 1.18 |
| 8 in | 0.70 | 1.05 | 1.40 | 1.75 | 2.09 |
| 10 in | 1.09 | 1.64 | 2.18 | 2.73 | 3.27 |
| 12 in | 1.57 | 2.36 | 3.14 | 3.93 | 4.71 |
| 16 in | 2.79 | 4.19 | 5.59 | 6.98 | 8.38 |
| 18 in | 3.53 | 5.30 | 7.07 | 8.84 | 10.60 |
| 24 in | 6.28 | 9.42 | 12.57 | 15.71 | 18.85 |
Tips before you buy
- Set the tube plumb and brace it before you fill, and fill in lifts, tamping or rodding to remove air pockets.
- Cut the tube to height with a handsaw, and set anchor bolts or post brackets into the wet concrete using a template.
- For decks and posts, the tube diameter and depth come from your plans or code (and must reach below the frost line in cold climates).
- Peel the tube off after a day or two where it is above ground, or leave it on for protection and finish.
Quick answers by size
Tools & materials for this project
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Frequently asked questions
How much concrete in a 12 inch sonotube?
About 0.79 ft³ per foot of height, so a 4 ft tube holds 3.14 ft³, which is about 5 bags of 80 lb (6 with 10% waste).
How many bags of concrete for an 8 inch tube?
A 8 inch tube holds 0.35 ft³ per foot of height. A 4 ft tube takes about 1.4 ft³, or 2.3 bags of 80 lb.
What size sonotube for a deck footing?
Deck footings are commonly 10 to 12 inches in diameter, set below the frost line, but your deck plans and local code decide the size.
Related calculators & guides
Estimates only. Results assume standard materials and simple shapes. Supplier products vary, and the final call on quantity, structure and safety is yours. Check your local code and your supplier's guidance before ordering.