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Concrete Column & Sonotube Calculator

Piers and columns are small pours repeated many times, which is exactly where estimating goes wrong. Work out one, multiply by the count, and buy a spare bag — the last hole always drinks more than the first. Round, square and rectangular are all covered below.

DiameterHeight
Round column / sonotube: diameter and height
SideHeight
Square column: side and height
LengthWidthHeight
Rectangular column: length, width and height
%

The formulas

Round: Volume = π × (Diameter ÷ 2)² × Height
Square: Volume = Side² × Height
Rectangular: Volume = Length × Width × Height

Worked example. A 10 in (250 mm) sonotube standing 4 ft (1.2 m) tall: π × 0.125² × 1.2 = 0.059 m³, about 2.08 ft³ — call it four 80 lb bags per pier. Six deck piers takes you to 0.35 m³ total, at which point a small ready-mix load starts to make sense.

Sonotube and pier quick reference (4 ft deep)

Tube diameter Volume each 80 lb bags each 25 kg bags each
8 in (200 mm) 1.40 ft³ / 0.040 m³ 3 4
10 in (250 mm) 2.18 ft³ / 0.062 m³ 4 6
12 in (300 mm) 3.14 ft³ / 0.089 m³ 6 8
16 in (400 mm) 5.59 ft³ / 0.158 m³ 10 14

Rough rule for a 10 in tube: one 80 lb bag per foot of depth. A 12 in tube takes about one and a half.

Getting piers right

  • Go below the frost line. 600 mm or more in most of the UK; anywhere from 12 to 48 inches in the US depending on the state. A pier that heaves in winter lifts whatever is sitting on it.
  • Bear on undisturbed soil. Backfill compacts under load no matter how well you tamp it.
  • Rod the pour. Narrow tubes trap air pockets against the form wall, and voids in a column are structural, not cosmetic.
  • Set anchors while it is wet, then leave them alone. Nudging a post base an hour later breaks the bond you just formed.

Frequently asked questions

How many bags of concrete per sonotube?

Roughly one 80 lb bag per foot of depth in a 10 inch tube, one and a half in a 12 inch. Enter your exact diameter and depth above and round up per hole rather than across the whole job.

How deep should deck piers be?

Below the local frost line, bearing on firm undisturbed ground. 600 mm is a common UK minimum; US depths vary widely by region, so check the local code rather than a general rule.

Do columns need rebar?

Structural columns always do — typically four vertical bars with ties, to the engineer’s design. Fence and deck piers often take a single central bar or none at all.

What mix for columns and piers?

C25 or stronger, which is 1:1.5:3 if you are mixing on site. Bagged high-strength mix suits small pier counts well.

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