Sonotube Calculator

A fiber form tube casts a right circular cylinder, so the concrete required is pi times the radius squared times the total height, counting both the buried shaft and the stub left above grade. A 12 inch tube has a radius of 0.5 foot and consumes about 0.785 cubic foot per foot of height, so a pier 48 inches deep with 6 inches of exposure takes roughly 3.5 cubic feet. Packaged mix yields about 0.6 cubic foot per 80 lb bag, so bag counts climb quickly and ready-mix delivery becomes economical near a half cubic yard of total placement. At about 150 lb per cubic foot, the same pier weighs close to 530 lb. Tube size is a bearing decision, not an aesthetic one. The end area of a 12 inch pier is about 113 square inches, or 0.785 square foot, so on soil with a presumptive bearing value of 1,500 to 2,000 pounds per square foot it can carry only roughly 1,200 to 1,600 pounds before the soil governs. Residential decks commonly use 8 to 12 inch tubes, and heavier loads are handled by adding a spread footing at the base rather than enlarging the shaft. Depth is set by frost rather than by load: footings must bear below the local frost line, which residential codes place anywhere from about 12 inches in the far south to 48 inches or more in northern states. Tubes must be braced plumb before placement because wet concrete will float and shift them, and an allowance of about 10 percent covers spillage and bulging at the base of an unsupported form.

The product that gave this calculation its common name came out of the paper packaging trade rather than construction. Sonoco Products Company was founded in 1899 in Hartsville, South Carolina, originally as the Southern Novelty Company, and built its business on spirally wound paper cones and tubes for the textile industry, where yarn was shipped on paper carriers. Spiral winding produces a lightweight tube with substantial hoop strength, and that combination turned out to be exactly what is needed to resist the outward pressure of fresh concrete. Sonoco introduced its fiber concrete form under the Sonotube name in the 1940s, and the term has since become a generic label for round column and pier forms regardless of manufacturer. Before fiber forms, a round concrete column had to be built from shaped wooden staves banded together or from rolled sheet metal, both slow and expensive enough that square and rectangular piers were the default for ordinary work. A disposable tube that could be cut to length on site, set in an augered hole, and stripped from the exposed portion afterward made the round pier practical for residential decks, porches, sign posts, and light foundations. Adoption was reinforced by the codification of frost-depth footing requirements in model building codes and later in the International Residential Code, which turned pier depth into a fixed regional requirement and made piers a repeatable, quantifiable item rather than a judgment call. Accessory bell-shaped footing bases were introduced later to add bearing area at the bottom of a tube without enlarging the shaft.