Sonotube Calculator

Sonotube calculator: concrete for round footings

A Sonotube calculator finds how much concrete fills one or more cardboard tube forms. A Sonotube is a brand name that has become shorthand for round fiber forms used for footings, piers, and columns. The math is the volume of a cylinder.

Cubic feet = pi × (diameter in inches ÷ 24)² × height in feet × number of tubes. Cubic yards = cubic feet ÷ 27. 60 lb bags = cubic feet ÷ 0.45, rounded up.

Worked examples

Example 1, the default tube. 12 in diameter, 4 ft tall, 1 tube.

• Radius: 12 ÷ 24 = 0.5 ft
• Volume: 3.1416 × 0.25 × 4 = 3.142 cubic feet
• Cubic yards: 0.116
• Bags: 3.142 ÷ 0.45 = 6.98, rounded up to 7

Example 2, a row of small piers. Six tubes, 8 in diameter, 3 ft tall.

• Volume per tube: 3.1416 × (8 ÷ 24)² × 3 = 1.047 cubic feet
• Total: 6.283 cubic feet, or 0.233 cubic yards
• Bags: 13.96, rounded up to 14

Example 3, a big deck project. Ten tubes, 12 in diameter, 4 ft tall.

• Total: 31.416 cubic feet, or 1.164 cubic yards
• Bags: 69.81, rounded up to 70
• Weight at 150 lb per cubic foot: about 4,712 lb of concrete

Concrete per foot of tube height

Diameter	Cubic feet per ft	60 lb bags per ft
8 in	0.349	0.78
10 in	0.545	1.21
12 in	0.785	1.75
16 in	1.396	3.10
18 in	1.767	3.93
24 in	3.142	6.98

Multiply the row by your height. A 12 in tube that is 4 ft tall takes 4 × 0.785 = 3.14 cubic feet.

What size tube to use

The diameter depends on the load, the soil, and the local code. The footing size calculator shows the bearing area a load needs. Match the tube’s cross-section area to that figure. For a deck post, the plans or the building department usually name the diameter and the depth. The depth must reach below the frost line where the ground freezes. Footings that hold up structures should be designed or confirmed by a licensed professional.

Setting the tubes

1. Dig the hole to the required depth, with the bottom on firm, undisturbed soil.
2. Cut the tube to length so the top sits where you need it. A hand saw works.
3. Set the tube plumb and brace it so it does not move during the pour.
4. Add reinforcing steel if the plans call for it. The rebar weight calculator helps with ordering.
5. Fill the tube in layers and tamp or rod it to remove air pockets.
6. Set the anchor or post bracket in the wet concrete, if the plans require it.
7. Keep the concrete damp as it cures, and strip the top of the tube after it sets.

Bags or ready-mix

Job	Better fit
1 to 4 tubes	Bags mixed by hand or in a mixer
5 to 8 tubes	Bags with a rented mixer, or a small ready-mix load
10 or more tubes	Ready-mix, if the delivery can reach the site

A bag of mix weighs 60 lb, and 70 bags weigh 4,200 lb, which is a lot to haul and mix.

Common mistakes

• Using the outside diameter of the tube when the inside is the pour size. The wall is thin, but check the product.
• Forgetting that the buried part of the tube counts toward the height.
• Rounding bags per tube instead of for the total.
• Pouring on loose soil at the bottom of the hole.
• Skipping the frost depth for your area.

Pouring day tips

Check the weather first. Concrete poured in heat sets faster, and concrete poured near freezing can be damaged. Wet the inside of the tube lightly if the day is hot. Mix bagged concrete to the consistency the label describes, since too much water weakens it. Pour steadily, and keep the tops level with a straightedge. If a tube floats or shifts, stop and reset before the concrete firms up. Wear gloves, eye protection, and a dust mask when handling dry mix.

Related calculators

The concrete weight calculator finds how heavy the pour is, and the fence post concrete calculator covers posts set in concrete without a tube.