Sonotube Calculator
A Sonotube is a cylinder, and the volume is πr²h. Here is what that means in bags.
A 12-inch Sonotube 4 feet tall needs 3.14 cubic feet of concrete — that is 6 bags of 80 lb with a 10 % waste allowance. The volume is worked out as πr²h, which for a 12-inch tube is exactly π cubic feet per foot of height.
Estimates only. Textured walls, high ceilings, sprayers and major colour changes can all increase the amount you need. Always buy slightly more than the calculation suggests.
Concrete needed by Sonotube size
| Pour | Volume | Cubic yards | Bags | Cheaper |
|---|---|---|---|---|
| 8" x 48", one tube | 1.5 cu ft | 0.1 cu yd | 3 × 80 lb | Bags |
| 10" x 48", one tube | 2.4 cu ft | 0.1 cu yd | 4 × 80 lb | Bags |
| 12" x 48", one tube | 3.5 cu ft | 0.1 cu yd | 6 × 80 lb | Bags |
| 12" x 48", six tubes | 20.7 cu ft | 0.8 cu yd | 35 × 80 lb | Ready-mix |
| 12" x 96", one tube | 6.9 cu ft | 0.3 cu yd | 12 × 80 lb | Bags |
| 16" x 48", one tube | 6.1 cu ft | 0.2 cu yd | 11 × 80 lb | Bags |
One tube each, 10 % waste. Sonotubes are named by their inside diameter, so a 12-inch tube really is 12 inches across.
A Sonotube is named by its inside diameter
This matters and catches people out. A "12-inch Sonotube" is 12 inches across inside, because that is the dimension that determines the concrete volume and the size of the post or pier it will hold.
The outside diameter is larger — a 12-inch tube is close to 12.5 inches outside. If you are excavating a hole to stand the tube in, allow for the outside dimension.
The volume formula is a cylinder: πr²h. For a 12-inch tube that works out pleasingly: a radius of 0.5 ft gives π × 0.25 × height, so 0.785 cubic feet per foot of height, or π cubic feet for a 4-foot tube.
Where Sonotubes are used, and why
A Sonotube is a cardboard form for casting a round concrete pier. It is the standard way to build deck, porch and pergola foundations in cold climates.
- Frost depth. The tube can be cut to any length, so it is easy to extend a pier from a footing below the frost line up to the deck framing above. That is its main advantage over a square footing in a trench.
- Neat finish. A cardboard tube leaves a smooth, round pier that looks deliberate above ground.
- Barrel and bracket. The top of the tube is usually fitted with a post base bracket set into the wet concrete, so the timber never touches the ground.
The tube must be braced before pouring. Wet concrete is heavy and the cardboard is not structural — an unbraced tube will bulge or blow out. Stake it in two directions at the top and check it is plumb.
Two practical points about pouring
Fill in lifts and rod the concrete. Drop concrete in from the top and it can segregate, with the aggregate settling and the paste rising. For tubes over about 4 feet, pour in 2-foot lifts and rod each one with a stick or a vibrator to remove air pockets.
Do not over-vibrate. Too much vibration makes the aggregate sink and leaves a weak, watery top. A few strokes per lift is plenty.
Also: leave the tube on. There is no need to strip it. It protects the concrete while it cures and it is hidden below ground or behind the deck anyway. Trying to peel it off risks damaging the surface you just cast.
Frequently asked questions
How much concrete for a 12-inch Sonotube?
For a 4-foot tube, 3.14 cubic feet before waste and about 3.46 with 10 %, which is 6 bags of 80 lb mix. A 12-inch tube holds 0.785 cubic feet per foot of height.
How many bags of concrete for a Sonotube?
A 12" x 48" tube takes about 6 bags of 80 lb. An 8" x 48" tube takes about 3, and a 16" x 48" tube takes about 10.
Is a Sonotube measured inside or outside?
Inside. A 12-inch Sonotube is 12 inches across internally, and the outside is about 12.5 inches, which matters when you are digging the hole.
Do I need to remove a Sonotube after pouring?
No. Leave it in place. It protects the concrete while curing and is either buried or hidden behind the structure. Removing it risks damaging the finished surface.