Concrete Cylinder Calculator
This calculator works out how much concrete you need to fill a cylindrical shape, such as a fence post hole, sonotube, pier, or round footing, so you can buy the right amount of premix and avoid running short mid-pour. Enter the diameter and depth in millimetres, centimetres, or metres, then set the number of cylinders you are pouring, since many jobs need several post holes at once. Choose a concrete density (standard, light, or heavy mix), a premix bag size from 20 kg to 40 kg, and a wastage allowance for spillage and uneven formwork, with 10 percent a typical default. Using the standard cylinder formula, volume equals pi times radius squared times height, the calculator applies your wastage allowance and density to return the total volume in cubic metres and litres, the estimated weight in kilograms, and the number of bags to buy. A breakdown shows the net volume per cylinder, the total across all cylinders, and a materials summary with the bag yield. A worked example using the 300 mm by 600 mm defaults is included, plus a note on whether hand-mixed bags or ready-mix concrete suits the volume entered. Use this before ordering materials for fence posts, deck piles, or retaining wall piers. These figures are estimates only, since actual use depends on hole shape and ground conditions, so always buy a spare bag or two.
1. Cylinder Dimensions
2. Concrete Options
Volume Breakdown
Materials Summary
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
- Science and Engineering Calculators
- Concrete Bags Needed Calculator
- Concrete Calculator
- Concrete Volume Calculator
- Cylinder Volume Calculator
- Granny Flat Rental ROI Calculator NZ: yield and Payback.
- Knock-Down Rebuild vs Renovate Calculator NZ: compare demolishing and rebuilding a new home against renovating your .
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.