Cylindrical Concrete Volume Formula & Bag Calculation
Round pier and Sonotube form volume is calculated using the geometric formula for a cylinder, accounting for diameter, depth, tube quantity, and optional post displacement:
Worked Example: Four 10-Inch Diameter Sonotubes at 4 Feet Depth
For a deck foundation requiring four 10-inch diameter piers at 4 feet (48 inches) depth:
- Radius = 5 inches; Area = π × 5² = 78.54 sq in
- Volume per tube = 78.54 × 48 in = 3,770 cu in (2.18 cu ft)
- Four tubes total = 8.72 cu ft (0.323 yd³)
- With 10% spillage allowance = 9.60 cu ft (0.356 yd³)
- 80 lb bag count: 9.60 ÷ 0.60 = 16 bags total (4 bags per tube)
80 lb & 60 lb Concrete Bags Per Sonotube (3 ft & 4 ft Depth)
Standard material requirements per single tube form including a 10% allowance for base flairing and minor excavation irregularities:
| Tube Diameter | 3 ft Depth Volume | 3 ft (80 lb Bags) | 3 ft (60 lb Bags) | 4 ft Depth Volume | 4 ft (80 lb Bags) | 4 ft (60 lb Bags) |
|---|---|---|---|---|---|---|
| 8 inch (20 cm) | 1.15 cu ft (0.04 yd³) | 2 bags | 3 bags | 1.54 cu ft (0.06 yd³) | 3 bags | 4 bags |
| 10 inch (25 cm) | 1.80 cu ft (0.07 yd³) | 3 bags | 4 bags | 2.40 cu ft (0.09 yd³) | 4 bags | 6 bags |
| 12 inch (30 cm) | 2.60 cu ft (0.10 yd³) | 5 bags | 6 bags | 3.45 cu ft (0.13 yd³) | 6 bags | 8 bags |
| 14 inch (35 cm) | 3.53 cu ft (0.13 yd³) | 6 bags | 8 bags | 4.71 cu ft (0.17 yd³) | 8 bags | 11 bags |
| 16 inch (40 cm) | 4.61 cu ft (0.17 yd³) | 8 bags | 11 bags | 6.15 cu ft (0.23 yd³) | 11 bags | 14 bags |
| 18 inch (45 cm) | 5.84 cu ft (0.22 yd³) | 10 bags | 13 bags | 7.78 cu ft (0.29 yd³) | 13 bags | 18 bags |
| 24 inch (60 cm) | 10.37 cu ft (0.38 yd³) | 18 bags | 24 bags | 13.82 cu ft (0.51 yd³) | 23 bags | 31 bags |
Note: One 80-lb bag yields approximately 0.60 cu ft; one 60-lb bag yields 0.45 cu ft. For piers 18" or larger, ready-mix transit delivery saves considerable labor.
Frequently Asked Questions
How many 80 lb bags of concrete fill a 10-inch Sonotube?
A 10-inch diameter Sonotube holds approximately 0.545 cubic feet of concrete per foot of depth. For a standard 4-foot frost depth pier (2.18 cu ft), you need exactly 4 bags of 80 lb pre-mixed concrete (each 80 lb bag yields 0.60 cu ft) with a 10% allowance.
How deep should Sonotubes be placed for frost protection?
Sonotubes should extend at least 6 to 12 inches below your local regional frost line to prevent frost heave from lifting deck or building footings. In northern US climates, this typically requires 3.5 to 4.5 feet of depth; southern regions often require only 1.5 to 2.5 feet.
Should I deduct the volume of a 4x4 or 6x6 post set into the tube?
If embedding a wood post directly into the concrete hole, a 4×4 post (actual 3.5×3.5 in) displaces 0.085 cu ft per foot of depth. A 6×6 post (actual 5.5×5.5 in) displaces 0.21 cu ft per foot. However, for structural deck piers, modern building codes recommend pouring concrete flush with the top of the Sonotube and securing posts with an elevated galvanized metal post base.
At what project size should I switch from bagged concrete to ready-mix delivery?
When your total pier volume exceeds 2.0 to 2.5 cubic yards (approximately 90 to 115 eighty-pound bags), mixing bags by hand or small drum mixer becomes extremely labor-intensive. Ready-mix transit delivery is faster, provides uniform 3,000+ PSI strength, and eliminates manual bagging.