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.
Related Calculators
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.
