How do I calculate how much concrete I need for a round column?

For a round (circular) column, the volume of concrete is: Volume = pi x radius squared x height. For example, a column with a 300 mm diameter (radius = 0.15 m) and a height of 1.2 m requires: 3.14159 x 0.15 x 0.15 x 1.2 = 0.0848 cubic metres. Add 10 percent waste to allow for spillage and overfill, giving approximately 0.0933 cubic metres needed. At 0.009 cubic metres per 20 kg premix bag, you would need about 11 bags.

How many 20 kg bags of concrete do I need for a fence post?

A standard fence post hole is typically 200 to 300 mm in diameter and 600 to 900 mm deep. For a 250 mm diameter hole at 750 mm deep, the concrete volume is approximately 0.0368 cubic metres (before waste; about 0.0405 cubic metres including 10 percent). A 20 kg premix bag yields roughly 0.009 cubic metres of mixed concrete, so you would need around 5 bags per post hole. For a 300 mm diameter hole at 900 mm deep, you need about 8 bags.

What is the density of concrete for weight calculations?

Normal-weight mixed concrete has a density of approximately 2,400 kg per cubic metre. This figure is used for structural calculations and load estimates. Lightweight concrete (used for insulation or reducing dead load) has a density of around 1,400 to 1,800 kg per cubic metre. Premix bag concrete after curing is also approximately 2,400 kg per cubic metre. For planning purposes, 1 cubic metre of standard concrete weighs about 2.4 tonnes.

Concrete Column Calculator

This calculator works out how much concrete you need for a round or square column, post or pier, so you can order the right amount of premix or ready-mix and avoid running short partway through a pour. Choose whether your column is round or square, then enter its diameter, or its width and depth, along with the column height or hole depth and how many columns you are pouring. Pick your premix bag size (20 kg, 25 kg or 40 kg) and a waste allowance for spillage and overfill, with 10 percent recommended. It returns the net volume with no waste, the total volume with waste added, the number of bags to buy, and the total cured weight, alongside a breakdown of the cross-section area, volume per column, waste allowance, and bags required both exact and rounded up. It suits fence posts, deck piles, foundation piers and freestanding columns, using the standard formulas of pi times radius squared times height for round columns and width times depth times height for square ones. Once your total passes about 1 cubic metre, the summary notes that ready-mix delivered by truck is usually cheaper and less labour than mixing dozens of bags by hand. Figures are estimates only. Actual concrete use depends on formwork tolerances, excavation accuracy and mix water content, so for structural or engineered columns, check with a licensed engineer.

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Standard formula Circular: V = π × r² × h. Rectangular: V = w × d × h. Concrete density 2,400 kg/m³.

1. Column Shape and Size

Column shape

Diameter (mm)

Column height / depth (mm)

Number of columns

columns

2. Concrete and Waste

Premix bag size

Waste / overfill allowance

Concrete density (kg/m³)

Concrete Required

Net Volume

0.0848

m³ (no waste)

Volume with Waste
0.0933
m³ to order / mix

Bags Needed

25 kg bags

Total Weight

204

kg (cured concrete)

Volume Breakdown

ShapeRound, 300 mm dia.

Cross-section area70686 cm2 (0.070686 m2)

Height1200 mm (1.200 m)

Number of columns1 column

Net volume (per column)0.0848 m3

Net volume (all columns)0.0848 m3

Waste allowance10% = 0.0085 m3

Total volume to order0.0933 m3

Bags and Weight

Bag size selected25 kg bag (~0.012 m3)

Volume per bag0.012 m3

Bags required (exact)7.78

Bags to buy (rounded up)8 bags

Concrete density2,400 kg/m3

Weight per column204 kg

Total concrete weight204 kg

Summary: Enter your column dimensions above.

How to Calculate Concrete Volume for a Column

The volume of concrete needed for a column depends on its cross-section shape and its height (or depth, if it is a foundation pier or fence post hole).

Round Column Formula

For a circular cross-section column, the formula is:

Volume = π × r² × h

Where r is the radius (diameter divided by 2) in metres and h is the height in metres. For example, a round column 300 mm in diameter and 1,200 mm tall has a volume of:

V = 3.14159 × 0.15 × 0.15 × 1.2 = 0.0848 m³

Add 10% for waste: 0.0848 × 1.10 = 0.0933 m³

Square or Rectangular Column Formula

For a square or rectangular cross-section, the formula is:

Volume = width × depth × height

For example, a 300 mm × 300 mm column at 1,200 mm tall:

V = 0.3 × 0.3 × 1.2 = 0.108 m³

Premix Bag Yields

Bag size	Approximate yield	Bags per 0.1 m³
20 kg	~0.009 m³	~12 bags
25 kg	~0.012 m³	~9 bags
40 kg	~0.019 m³	~6 bags

Yields are approximate. Actual yield depends on water content, mixing thoroughness, and aggregate size. Always add a waste allowance of at least 10% to avoid running short.

Worked Example: Fence Post Holes

You are setting 4 fence posts with round holes 250 mm in diameter and 800 mm deep.

Radius = 0.125 m
Volume per hole = π × 0.125² × 0.8 = 0.03927 m³
Total for 4 holes = 0.03927 × 4 = 0.1571 m³
With 10% waste = 0.1728 m³
At 0.012 m³ per 25 kg bag = 15 bags (rounded up)

This calculator handles all these steps automatically. Just enter your dimensions, number of columns, bag size, and waste factor.

Tips for Concrete Column Work

For fence posts in NZ soil, the post hole should be at least one-third of the post length deep and wider than the post to allow for concrete surround.
Round formwork (cardboard tube forms or Sonotube) works well for exposed decorative columns. Square timber formwork is easier for structural piers.
Mix premix bags in a barrow or mechanical mixer. Add water gradually. Concrete should be workable but not sloppy.
For large columns or multiple pours, consider ordering ready-mix concrete in cubic metres, which is more cost-effective above about 1 m³.
Use a vibrator or rod the concrete to eliminate air voids, especially in deep, narrow holes.

When to Use Ready-Mix Instead of Bags

Premix bags are practical for small volumes, typically up to about 0.5 to 1.0 m³. Above that, ready-mix concrete delivered by truck is usually cheaper per cubic metre and far less labour intensive. Ready-mix can be ordered in 0.2 m³ increments from most suppliers. Ask for a standard 20 MPa mix for footings and posts, or 30 MPa for structural columns.

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Method: Round column volume calculated as V = π × r² × h. Rectangular column volume as V = w × d × h. All dimensions converted from mm to metres before calculation. Bag count rounded up to nearest whole bag. Concrete weight = volume (with waste) × density (kg/m³). Standard concrete density 2,400 kg/m³ per NZS 3101.

This calculator provides estimates only. Actual concrete volume depends on formwork tolerances, excavation accuracy, and mix water content. For structural or engineered columns, consult a licensed engineer. Premix bag yields are approximate and vary by manufacturer.