Round Column Calculator

A concrete column is a vertical structural member that transfers load from a beam, deck, or roof above down to a footing below. Round columns are formed inside cylindrical tubes (commonly Sonotube brand cardboard forms) and reinforced with vertical rebar. Five common residential uses cover most jobs:

• Deck support posts — typically 8–12 in (20–30 cm) diameter, set on a footing below the frost line.
• Fence and gate posts — 6–10 in (15–25 cm) diameter, often filled around a steel or wood post.
• Mailbox and sign posts — 6–8 in (15–20 cm) diameter, shallow set.
• Porch and pergola pillars — 12–16 in (30–40 cm) diameter, often dressed with stone or brick veneer.
• Pole barn / pier piles — 16–24 in (40–61 cm) diameter, structural columns supporting roof load.

A round concrete column is a cylinder. Volume equals π × radius² × height. Convert radius from inches to feet (divide by 12 twice) so the result comes out in cubic feet, then divide by 27 for cubic yards.

Worked example: a 12 in (30 cm) diameter column, 8 ft (2.4 m) tall: π × (12 ÷ 24)² × 8 = 6.28 ft³ ÷ 27 = 0.23 cubic yards. Six identical columns total 1.40 yd³ with 5% waste — order ~1.5 yd³ ready-mix.

Drag the diameter and height sliders to resize the column. The rebar cage and footing pad scale with the dimensions, and the concrete volume per column updates live.

Sonotube is a brand of cardboard concrete forming tube. The math is identical for any round column form — just enter the inside diameter. Common sizes and the concrete required per linear foot:

Volumes shown without waste factor. Add 5% for typical sonotube installations.

Vertical rebar resists the bending and tension forces in a loaded column. Standard residential reinforcement uses 4 vertical bars (#4 or #5) tied with #3 horizontal stirrups every 12 in (30 cm). The rebar cage extends from the footing dowels up through the column, with 3 in (7.6 cm) of concrete cover on all sides.

• Small columns (8–12 in / 20–30 cm): 4 × #4 vertical bars
• Medium columns (14–18 in / 36–46 cm): 4 × #5 vertical bars
• Large or load-bearing columns (20+ in / 51+ cm): 6+ × #5 or #6 bars, engineered design required

Round columns pour quickly because the form is clean and contained. Pour the footing first, place vertical rebar dowels, then set the sonotube and brace it plumb. Pour concrete in 12–18 in (30–46 cm) lifts, vibrating each lift to release trapped air. Strip the cardboard form 24–48 hours after pouring once the concrete reaches initial set.