Sonotube Calculator
This sonotube calculator works out how much concrete a form holds, how many bags that is, and how many tubes a deck or fence run needs.
Concrete per Sonotube
Sonotube with a Footing Pad
Deck and pier footings are often a tube sitting on a wider pad. This adds both together.
How Many Tubes Does the Run Need?
Enter the length of the deck beam or fence line and the spacing your plan calls for.
How to use the Sonotube Calculator
Pick the tube diameter off the form, Sonotube prints it on the side, set how deep the pour goes, and enter how many tubes you have. The calculator returns the concrete for the whole set, not one tube.
Use Tube + Pad when the footing flares out at the bottom, and How Many to work the count out from a beam or fence line.
Sonotube formula
A filled Sonotube is a solid cylinder, not a hollow tube, the cardboard form is what is hollow, and it comes off:
volume = π × (diameter ÷ 2)² × depth × quantity
With a footing pad underneath:
- Round pad:
π × (pad diameter ÷ 2)² × pad thickness - Square pad:
pad width² × pad thickness total per footing = tube + pad
For the layout: footings per row = floor(run ÷ spacing) + 1, because a run needs a post at both ends.
Weight and bag counts use a pre-mixed density of 133 lb/ft³, matching every other tool on the site.
| Diameter | Concrete per foot of depth | 80 lb bags per foot |
|---|---|---|
| 8 in | 0.35 ft³ | 0.6 |
| 10 in | 0.55 ft³ | 0.9 |
| 12 in | 0.79 ft³ | 1.3 |
| 16 in | 1.40 ft³ | 2.3 |
Worked example
Four 12 inch tubes, each filled 4 feet deep:
- Radius: 6 in = 0.5 ft. Area: π × 0.5² = 0.785 sq ft
- One tube: 0.785 × 4 = 3.14 ft³
- Four tubes: 12.57 ft³ = 0.47 cubic yards
- Weight: 12.57 × 133 = 1,671 lb, so 21 bags of 80 lb
Just under half a yard, right on the line where bagging stops being sensible and a short-load delivery starts to look better.
What the result means
The figure is the concrete that goes inside the forms. It does not include the gravel base some footings sit on, and it assumes the tube is filled to the top of the form.
Sonotube is a form, not the structure. Depth, diameter, rebar and whether the footing needs to reach below the frost line are set by your local building code and your span tables, this calculator prices the pour, it does not size the footing.
Common mistakes
- Treating it as a hollow tube. The cardboard is the form. You fill the whole cylinder, so use the solid-cylinder volume, not an annulus.
- Measuring the hole instead of the tube. The excavation is always wider. Concrete only fills the form.
- Forgetting the frost line. Footing depth is a code requirement, not a preference. In cold regions it can be four feet or more, which changes the concrete a lot.
- Counting posts instead of spans. A 24 ft run at 6 ft centres takes five posts, not four, there is one at each end.
Frequently asked questions
How many bags of concrete for a 12 inch sonotube?
A 12 inch tube holds about 0.79 cubic feet per foot of depth, which is roughly 1.3 bags of 80 lb per foot. A 4 foot deep tube needs about 6 bags of 80 lb, or 8 bags of 60 lb.
How many bags of concrete for an 8 inch sonotube?
An 8 inch tube holds about 0.35 cubic feet per foot of depth, so roughly 0.6 bags of 80 lb per foot. A 4 foot tube takes about 3 bags of 80 lb.
What size Sonotube do I need?
That comes from the load the post carries and your soil, and it is set by your local building code and span tables rather than by rule of thumb. Deck piers commonly run 8 to 12 inches; heavier structures go larger. Check the plans or ask your building department.
How deep should a Sonotube footing be?
Below the frost line for your area, which your local code specifies. That can be a foot in mild climates and four feet or more in cold ones. Footings that stop short of it heave and lift the structure.
Do you leave the cardboard tube in the ground?
The buried portion stays; it does no harm and decays over time. The part above grade is usually stripped once the concrete sets, because damp cardboard left against a pier looks poor and holds moisture.
How many sonotubes do I need for a deck?
Divide the beam run by the spacing your span table allows and add one, because a post is needed at each end. A 24 foot beam at 6 foot centres needs 5 footings per beam line. Use the How Many tab and set the row count to the number of parallel beams.
Sources and assumptions
Volumes are exact geometry for a solid cylinder, plus a round or square pad where one is entered. Weight and bag counts use a pre-mixed density of 133 lb/ft³, the same figure the other calculators on this site use, so numbers agree across tools.
Diameters listed are the sizes Sonotube forms are commonly sold in. For a non-standard diameter, use the Column panel on the Concrete Calculator.
This tool estimates concrete. It does not size footings. Diameter, depth, reinforcement and frost-line requirements are set by your local building code and the structure’s loads.