Sonotube Calculator: Concrete for Round Column Forms

A cardboard tube form turns a round hole into a clean concrete column, and it is the standard way to pour deck footings, porch posts, pier foundations, mailbox posts and carport supports. The volume is a straightforward cylinder — but the two mistakes that cost people a second trip are measuring the tube rather than the pour, and forgetting the footing pad underneath.

Concrete per foot of tube

Volume scales with the square of the diameter, so stepping up one tube size adds far more concrete than it looks. Going from an 8 in tube to a 12 in tube roughly doubles the concrete for the same height.

In metric, per metre of height:

Bag counts assume the usual yields — about 0.6 ft³ from an 80 lb bag and about 0.01 m³ from a 20 kg bag. Always round up to whole bags.

Don’t forget the footing pad

Most deck and pier footings need a wider base under the tube to spread the load into the soil. That pad is extra concrete and it is easy to leave out of the estimate:

• A square pad — work it out as a rectangular slab and add it to the tube volume
• A belled base, flared out at the bottom of the hole, holds noticeably more than the tube alone; treat it as a second, wider cylinder for a rough figure and expect it to run over
• A pre-formed footing base that clips to the tube will state its own volume on the packaging

Pad size, footing depth and reinforcement are all set by local code, and they vary a lot by soil type and climate. Check before you dig.

Setting the tubes

• Brace before you pour. A filled tube is heavy and wants to float, lean and twist. Stake and brace every tube plumb, and check plumb again after the first lift.
• Pour in lifts and rod each one. Fill in roughly 600 mm lifts, rodding or vibrating each to release trapped air. A tall column poured in one go will honeycomb.
• Don’t overfill before you level. Bring the last lift up slowly to the mark — screeding a round top is fiddly, and concrete that spills down the outside is wasted.
• Set anchors while it is plastic. Post bases and anchor bolts go in immediately after the pour, before the concrete stiffens.
• Strip the cardboard. Cut the form away once the concrete has cured; left in the ground it holds moisture against the column.

Tube pours vs ready-mix

Tube footings are usually a bagged-concrete job — a handful of footings rarely reaches a ready-mix truck’s minimum order, which is often 1 yd³ or 1 m³. But the arithmetic turns over faster than people expect. Twelve 12 in footings at 4 ft each is about 38 ft³, or 1.4 yd³ — well past the point where mixing by hand stops being sensible and a truck or a trailer mixer is worth the call.

Use it on the jobsite

The Concrete Calculator app for iOS has this same calculator plus saved projects, history, four more calculator types and full offline support for when the site has no reception. Get it on the App Store.

Pouring something other than a tube? The circular column calculator is the same maths with unit defaults suited to structural columns, and the post hole calculator subtracts the volume a post takes up when you are setting one in the hole.

Sonotube is a registered trademark of Sonoco Products Company. This calculator is not affiliated with or endorsed by Sonoco — it works with any round column form.