What is a Concrete Tube Form?
A concrete tube form (widely recognized by the brand name Sonotube®) is a heavy-duty, spirally wound cardboard cylinder used to cast concrete columns, piers, and footings. These tubes are an essential staple in construction because they allow builders to pour perfectly cylindrical, smooth concrete pillars both above and below ground without building complex wooden formwork.
Tube forms are commonly used for:
- Deck supports and gazebo footings
- Mailbox posts and flagpoles
- Lamp post bases
- Structural columns in residential and commercial buildings
How to Calculate Volume for Concrete Tubes
Concrete is typically sold in cubic yards (if ordering a truck) or cubic feet (if buying bags). To calculate how much concrete you need to fill a tube form, you are simply calculating the mathematical volume of a cylinder.
The Formula
To find the volume of the tube in cubic yards:
- Divide the Tube Diameter (in inches) by 2 to find the Radius.
- Convert the Radius to feet (divide by 12).
- Square the radius (multiply it by itself).
- Multiply the squared radius by Pi (π ≈ 3.14159). This gives you the area of the tube's opening in square feet.
- Multiply that area by the Tube Height (ft) to get the total Cubic Feet.
- Divide the total cubic feet by 27 to convert to Cubic Yards.
Volume (yd³) = (π × ( (Diameter ÷ 2) ÷ 12 )² × Height) ÷ 27
Example Calculation
Let's calculate the volume for a standard deck footing using a tube that is 12 inches in diameter and 4 feet tall.
- Find Radius in feet:
12 ÷ 2 = 6 inches. Then6 ÷ 12 = 0.5 ft. - Square the Radius:
0.5 × 0.5 = 0.25. - Find Area:
3.14159 × 0.25 ≈ 0.785 square feet. - Find Cubic Feet:
0.785 sq ft × 4 ft = 3.14 cubic feet. - Find Cubic Yards:
3.14 ÷ 27 ≈ 0.116 cubic yards.
Because 3.14 cubic feet is a very small volume, you would likely use bagged concrete. Since an 80 lb bag yields about 0.60 cubic feet, you would need 3.14 ÷ 0.60 = 5.23 bags (so buy 6 bags).
Best Practices for Using Tube Forms
When working with cardboard tube forms, preparation is everything:
- Bracing: Cardboard tubes are light, but wet concrete is incredibly heavy. A 12-inch tube filled 4 feet high holds over 450 lbs of concrete. If the tube is not rigidly braced with 2x4s before pouring, it will easily tip over or shift out of plumb.
- Moisture: While the inside of a quality tube is coated to resist moisture from the wet concrete, the outside is just cardboard. Do not leave them in a wet trench or exposed to rain for days before pouring, or they will degrade and burst during the pour.
- Vibration: Consolidate the concrete as you pour by tapping the outside of the tube with a rubber mallet or plunging a piece of rebar up and down inside. This removes trapped air bubbles (honeycombing) that weaken the final pillar.