How It Is Calculated
This calculator estimates the volume of concrete needed to fill one or more sonotubes (cylindrical forms) and the number of bags required. The calculation follows these steps:
- Convert the diameter from inches to feet: D = d / 12.
- Calculate the radius in feet: R = D / 2.
- Compute the cross-sectional area in square feet: A = π × R².
- Compute the volume per tube in cubic feet: Vtube = A × H.
- Compute the total volume without waste in cubic feet: Vtotal = Vtube × N.
- Apply the waste factor: Vwaste = Vtotal × (1 + W / 100).
- Convert to cubic yards: Y = Vwaste / 27. Round to 2 decimal places.
- Cubic feet output: Vwaste rounded to 1 decimal place.
- Number of 60 lb bags: B60 = ceil(Vwaste / 0.45).
- Number of 80 lb bags: B80 = ceil(Vwaste / 0.6).
Vwaste = π × (d / 24)² × H × N × (1 + W / 100)
Assumptions
- The sonotube is a perfect cylinder.
- Concrete fills the tube completely with no voids.
- Waste factor accounts for spillage, over-excavation, and variations in form size.
- Bag yields: 60 lb bag yields 0.45 ft³, 80 lb bag yields 0.6 ft³. These are typical values; actual yields may vary by mix and manufacturer.
- All measurements are in US customary units.
Worked Example
Suppose you have a sonotube with a diameter of 12 inches, a height of 4 feet, and you need 1 tube with a 10% waste factor.
- Convert diameter to feet: D = 12 / 12 = 1 ft.
- Radius: R = 1 / 2 = 0.5 ft.
- Area: A = π × (0.5)² ≈ 0.7854 ft².
- Volume per tube: Vtube = 0.7854 × 4 = 3.1416 ft³.
- Total without waste: Vtotal = 3.1416 × 1 = 3.1416 ft³.
- With waste: Vwaste = 3.1416 × (1 + 10/100) = 3.1416 × 1.1 ≈ 3.4558 ft³.
- Cubic yards: Y = 3.4558 / 27 ≈ 0.1280 yd³, rounded to 0.13 yd³.
- Cubic feet: 3.4558 rounded to 3.5 ft³.
- 60 lb bags: ceil(3.4558 / 0.45) = ceil(7.6796) = 8 bags.
- 80 lb bags: ceil(3.4558 / 0.6) = ceil(5.7597) = 6 bags.
Practical Tips
- Always add a waste factor (typically 5–10%) to account for spillage and uneven filling.
- Use a sonotube that is slightly larger than the required diameter to allow for easy placement and alignment.
- Ensure the tube is level and securely braced before pouring concrete.
- For multiple tubes, consider ordering ready-mix concrete if the total volume exceeds 1 cubic yard.
- If you need to calculate concrete for a slab or foundation, try the Concrete slab calculator or Concrete foundation calculator.
- To estimate the weight of the concrete, use the Weight concrete calculator.
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