How this instrument works
A concrete tube form is a round fiber or cardboard cylinder — Sonotube is one well-known brand, but plenty of generic equivalents do the identical job — used to shape wet concrete into a cylindrical footing, pier or post base before the ground is backfilled around it. Builders set the tube in an excavated hole, pour concrete inside it, and once the concrete cures the tube is stripped away or, for below-grade sections, simply left to rot in place, leaving a clean round column of concrete behind.
The volume inside any such tube is the ordinary cylinder formula: pi times the radius squared times the height. This instrument takes the diameter in inches, which is how tube forms are actually sold and specified on a supply-house shelf, halves and converts it to a radius in feet by dividing by 24, then multiplies by height and pi and divides by 27 to hand back cubic yards, the unit ready-mix concrete is ordered in.
Because a tube form encloses a fixed, known shape, this is one of the few concrete-volume estimates that needs no rule-of-thumb fudge factor the way an irregular footing or stair pour does — the geometry is exact, so the only real-world adjustment is rounding the final figure up to whatever minimum order or full-bag increment your supplier sells.
- Enter Tube form diameter (in) — the form's inside diameter as printed on the tube or its packaging, ten inches by default.
- Enter Tube form height (ft) — how tall the form stands once set in the hole, including any portion left above grade.
- Read Concrete needed (cubic yards) directly; it recalculates the instant either field changes.
- For more than one footing or pier of the same size, multiply the result by the number of tubes, or run the instrument once per tube if sizes differ.
- Round the final figure up to your supplier's minimum delivery or bag-count increment — a tube form is an exact shape, but concrete is not sold in arbitrary fractions of a yard.
Worked example — 10 in diameter, 6 ft tall tube form
Enter 10 into Tube form diameter (in) and 6 into Tube form height (ft) — a common size for a deck post footing. Radius works out to 10/24 = 0.41667 ft, so volume = π × 0.41667² × 6 = 3.2725 ft³.
Concrete needed (cubic yards) reads 0.12120 — 3.2725 ft³ divided by 27. A single tube like this uses barely an eighth of a yard, which is why most jobs pour several footings from one small ready-mix order or a handful of pre-mixed bags rather than a full truck.
Questions
Is a concrete tube form the same thing as a Sonotube?
Sonotube is one manufacturer's brand name for a round fiber concrete-forming tube, similar to how a facial tissue is sometimes called a Kleenex regardless of maker. This instrument works for any round tube form — Sonotube or a generic equivalent from another supplier — because the volume only depends on the form's inside diameter and height, not on which company made it.
How is this different from sizing a structural concrete column?
A tube form pours a temporary or partly buried shape — commonly a pier, post base or footing set below grade — sized here by geometry alone. A structural column calculation instead sizes a permanent, load-bearing member and typically also checks reinforcement and bearing capacity, not just concrete volume, even though both start from the identical cylinder-volume formula.
How is this different from an ASTM concrete test cylinder?
A standard concrete test cylinder is a small 4×8 in or 6×12 in mold used under ASTM C31/C39 to sample wet concrete for a compressive-strength test, not a structural or footing shape at all. This instrument instead sizes full-scale tube forms — commonly 8 to 24 inches in diameter and several feet tall — for piers, posts and footings, not laboratory test specimens.
Why divide the diameter by 24 instead of 12?
Two conversions happen in one step: dividing by 2 turns a diameter into a radius, and dividing by 12 turns inches into feet. Doing both at once means dividing by 2 × 12 = 24, which is why a 10 in diameter tube becomes a 0.41667 ft radius directly, without a separate intermediate step.
How much should I add for waste or spillage?
This instrument returns the exact geometric volume of the tube itself, with no allowance for spillage, overfill above the form, or a form that settles slightly wider than its nominal size during the pour. Many contractors add roughly 5–10% on top of the calculated figure, or simply round up to their supplier's next full bag or minimum delivery increment.
Can I use this for multiple footings on the same job?
Yes — calculate one tube's volume here, then multiply by the number of identical footings to get the job total, or run the instrument once per tube if diameters or heights vary across the site. Concrete for several small footings is often ordered as one combined delivery rather than tube by tube.