Concrete
Round Column and Sonotube Calculator
Round forms (often called sonotubes) are used for deck piers, post bases and columns. Enter the diameter, height and number of piers to get total concrete volume.
Formula
Volume of a cylinder = π × r² × h. Radius in feet = diameter (in) ÷ 24. Total = per-pier volume × count. Cubic yards = ft³ ÷ 27.
Worked example
Six 12 in piers, each 4 ft deep: radius = 0.5 ft, so each holds π × 0.5² × 4 = 3.14 ft³. Six piers = 18.85 ft³ = 0.70 yd³, or 32 bags of 80 lb mix (18.85 ÷ 0.60 = 31.4, rounded up).
Assumptions and limits
- Cylindrical form with uniform diameter; belled or flared footings need extra volume.
- No waste allowance is included; add 5% to 10% if the holes are irregular.
- For estimation only. A licensed Professional Engineer (PE) must design and stamp any structural, foundation or life-safety element.
Frequently asked questions
How deep should deck piers go?
Footings must extend below the local frost depth set by your building department. The IRC requires footings to be below the frost line unless frost-protected; local depths range from a few inches to more than 4 ft.
What size sonotube do I need?
Pier diameter depends on the load and the soil bearing capacity. Many deck guides use 8 to 12 in tubes over a wider footing, but your local code or engineer sets the size.
Does the hole outside the tube need concrete?
No. The tube holds the concrete; the hole around it is backfilled with soil. Only footings poured directly into an unformed hole consume extra concrete.