What is a sonotube used for?

A sonotube (or concrete tube form) is a round cardboard form used to pour a cylindrical concrete footing or pier — commonly for deck posts, fence posts, mailbox posts and small structural columns. It stays in place, or is peeled away, once the concrete has cured.

How many bags of concrete do I need for a sonotube?

Work out the cylinder volume (pi x radius² x height), multiply by the number of tubes, add a waste allowance, then divide by your bag's yield. For example, four 12 in x 1.2 m tubes need about 0.377 m³ including 7.5% waste — roughly 23 bags of 25 kg pre-mix (0.017 m³ yield each).

What frost depth should I use for a fence or deck post footing in the UK?

The UK rarely has a single fixed national frost-depth figure the way colder climates do, but NHBC guidance commonly points to a minimum foundation depth of 450 mm to guard against frost heave, deeper in clay soils prone to shrink-swell. Check your local building control officer for the specific requirement in your area.

Concrete Tube Formwork Calculator (Sonotube) — Volume for Round Forms | UK

Sonotube Calculator: Concrete Volume for Round Column Forms

Sonotubes — round cardboard forms used to pour concrete for deck posts, fence posts and pier footings — need a precise volume, since a form that runs short mid-pour is a real problem and over-ordering ready-mix wastes money. This calculator works out that volume from your tube's diameter, height and how many tubes you are pouring, then converts it into either a bag count or a cement:sand:aggregate mix split, plus a material cost estimate. It covers the standard preset diameters as well as custom sizes, so it works equally for a single deck post or a full run of fence-post footings.

How to Use This Calculator

Choose a standard tube diameter from the preset list (6-24 in / 15-61 cm), or enter a custom diameter.
Enter the tube height — the depth the concrete fills — and its unit.
Set the number of identical tubes you are pouring.
Adjust the waste allowance if needed — 5-10% is standard.
Choose bags or a custom mix ratio, then read off the total volume, bag count or mix split, and estimated cost.

The Math Behind the Calculation

A tube is a cylinder, so its volume is pi x radius² x height (radius = diameter ÷ 2). That per-tube volume is multiplied by the number of tubes, then inflated by the waste allowance to give the volume you should actually order.

Worked example, this calculator's own defaults: a 12 in (0.3048 m) diameter tube, 1.2 m tall. Volume = pi x 0.1524² x 1.2 ≈ 0.088 m³ per tube. For 4 tubes, the raw total is 4 x 0.088 = 0.35 m³. Adding a 7.5% waste allowance: 0.35 x 1.075 ≈ 0.377 m³ to order — around 23 bags of 25 kg pre-mix (0.017 m³ yield each).

Pro Tips

At a typical UK ready-mix concrete price of around £125 per m³, this calculator's own default example (four 12 in x 1.2 m tubes, 0.377 m³ incl. 7.5% waste) works out to roughly £47 in material — small loads often carry a minimum-order surcharge, so check this against a current quote before ordering.

Pour footings below your local frost depth (the depth to which winter ground-freezing reaches) — a shallower footing can heave and crack over time, so check your local building code for the required minimum depth.
Standard tube diameters run from 6 in (15 cm), typically used for fence posts, up to 24 in (61 cm) or larger for structural piers — a bigger load needs a wider tube, not just a deeper one.