How do I calculate the volume of concrete for a cylinder?

Use the cylinder volume formula: Volume = pi x radius squared x height (V = pi r2 h). For a post hole with a diameter of 300 mm (radius = 0.15 m) and a depth of 600 mm (0.6 m), the volume is 3.14159 x 0.15 x 0.15 x 0.6 = approximately 0.042 cubic metres (42 litres). Multiply that by the concrete density (typically 2,400 kg per cubic metre) to get the weight, then divide by your bag size to find how many bags you need.

How many 20 kg bags of premix concrete fill a post hole?

A single 20 kg bag of standard premix concrete yields approximately 10 litres (0.010 cubic metres) of mixed concrete. For a 300 mm diameter hole that is 600 mm deep, the volume is about 42.4 litres, requiring roughly 5 bags (allowing for the volume displaced by the post itself). Always buy one or two extra bags to account for wastage and spillage. For sonotubes or larger diameter fills, the number of bags rises quickly, so consider ordering ready-mix concrete for holes larger than about 0.1 cubic metres.

What is a sonotube and how much concrete does it hold?

A sonotube (or concrete form tube) is a cylindrical cardboard tube used as a mould for pouring concrete columns, piers, and footings. Common sizes in New Zealand range from 100 mm to 600 mm diameter. A 200 mm sonotube that is 1 metre tall holds approximately 31.4 litres of concrete (0.031 m3), which weighs about 75 kg and requires 4 bags of 20 kg premix. A 300 mm sonotube at the same height holds about 70.7 litres, requiring 8 bags. Use this calculator to find the exact volume for any diameter and height.

Concrete Cylinder Calculator

Calculate the volume, weight, and number of premix bags needed to fill a concrete cylinder such as a post hole, sonotube, pier, or round footing. Enter the diameter and depth in your preferred unit and the calculator updates instantly.

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Reviewed June 2026 Standard cylinder volume formula (V = πr²h). Concrete density 2,400 kg/m³. Premix bag yield 10 L per 20 kg bag.

1. Cylinder Dimensions

Unit of measurement

Diameter (outer width of hole or tube)

Depth (height) (length of the cylinder)

Number of cylinders

2. Concrete Options

Concrete density

Premix bag size

Wastage allowance

Concrete Required

Volume (m³)
-
Including wastage

Volume (litres)

Including wastage

Weight

kg (wet concrete)

Bags Required

x 20 kg bags

Volume Breakdown

Diameter-

Radius-

Depth (height)-

Number of cylinders-

Net volume (per cylinder)-

Total net volume-

Wastage allowance-

Total volume (with wastage)-

Materials Summary

Concrete density-

Total volume-

Estimated weight-

Bag size-

Yield per bag (approx)-

Bags to buy-

Note: Enter your dimensions above.

Worked Example (default values)

Inputs: Diameter = 300 mm, Depth = 600 mm, 1 cylinder, 10% wastage, 20 kg bags.

Radius = 300 / 2 = 150 mm = 0.150 m

Net volume = π × 0.150² × 0.600 = 3.14159 × 0.0225 × 0.6 = 0.0424 m³ (42.41 L)

With 10% wastage = 0.04241 × 1.10 = 0.0467 m³ (46.65 L)

Weight = 0.0467 × 2,400 = 112 kg

Bags = ceil(46.65 / 10) = 5 bags of 20 kg (each yields ~10 L)

How the Concrete Cylinder Formula Works

The volume of a cylinder is calculated using the standard geometric formula:

V = π × r² × h

Where V is volume (m³), r is the radius in metres (half the diameter), and h is the height (depth) in metres. Pi (π) is approximately 3.14159.

Once you have the net volume, the calculator adds your chosen wastage allowance. For most small jobs, 10% wastage is appropriate. Larger pours or sites with uneven formwork may need 15%.

The weight is estimated by multiplying volume by the density of mixed concrete (typically 2,400 kg/m³ for standard mixes). This helps you assess whether you need a barrow or mixing equipment.

Premix Bag Yield

A standard 20 kg bag of premix concrete yields approximately 10 litres (0.010 m³) of mixed concrete. This is widely used as a planning guide across New Zealand and Australian building suppliers. Larger bags yield proportionally more, but the 20 kg bag is the most common for small jobs such as post holes and footings.

Bag Size	Approximate Yield	Typical Use
20 kg	10 litres (0.010 m³)	Post holes, small repairs
25 kg	12.5 litres (0.013 m³)	Footings, piers
30 kg	15 litres (0.015 m³)	Medium slabs, columns
40 kg	20 litres (0.020 m³)	Large pours

Note: bag yields vary slightly by manufacturer and mix design. Check the bag label for the stated yield if it differs from the above.

Common Cylinder Sizes and Bag Counts

Diameter	Depth	Volume (m³)	Volume (L)	20 kg bags (10% wastage)
100 mm	300 mm	0.0024	2.4	1
150 mm	450 mm	0.0080	8.0	1
200 mm	600 mm	0.0188	18.8	3
300 mm	600 mm	0.0424	42.4	5
300 mm	1,000 mm	0.0707	70.7	8
450 mm	900 mm	0.1431	143.1	16
600 mm	1,200 mm	0.3393	339.3	38

For cylinders larger than about 0.1 m³, ordering ready-mix concrete by the cubic metre is usually more economical and practical than hand-mixing bags.

Tips for Pouring Concrete Cylinders

Always check the hole diameter at the base as well as the top. Holes often taper or widen underground, so the actual volume can exceed your estimate by 10 to 20 percent.
For fence posts and structural poles, a minimum post depth of one-third to one-half of the above-ground height is recommended in New Zealand practice.
Sonotubes (cardboard concrete forms) are available from building suppliers in diameters from 100 mm to 600 mm and are commonly used for deck piles and retaining wall piles.
Wet concrete in a cylinder exerts hydrostatic pressure on the form. For deep pours, ensure the form is securely braced or held in place while the concrete cures.
Add your wastage allowance before buying bags. Running short mid-pour compromises the structural integrity of the column.

Related Calculators

Sources and method: Cylinder volume formula V = πr²h (standard geometry). Concrete density 2,400 kg/m³ per NZS 3104:2003 (Specification for concrete production). Premix bag yield based on manufacturer guidance (approximately 0.010 m³ per 20 kg bag for standard mixes). Wastage factors based on typical site practice.

This calculator provides estimates only. Actual concrete requirements depend on site conditions, hole shape, and mix design. Always buy extra bags to allow for spillage and uneven formwork. For structural work, consult a licensed building practitioner or engineer.