The amount of concrete needed for a fence post depends on the hole dimensions, the portion of the hole filled with concrete and the space taken up by the post. Counting posts alone won't tell you how many bags to buy.
1. Measure each post hole
For a round hole, measure its diameter and the planned concrete-fill depth. Convert millimetres to metres before calculating. If the holes differ, calculate them separately. The appropriate hole dimensions depend on fence height, wind exposure, ground conditions and the post or fence system: this guide is a quantity estimate, not a footing design.
2. Subtract the post's volume
Concrete per hole (m³) = π × (hole diameter ÷ 2)² × fill depth − post cross-sectional area × embedded length within the concrete. For a square post, cross-sectional area is width × width. Use consistent metre units throughout.
Worked example: six square posts
Suppose there are six holes, each 300 mm diameter, with 600 mm concrete-fill depth, and each contains a 100 × 100 mm square post extending through that fill depth.
- Round hole volume: π × 0.15² × 0.6 ≈ 0.04241 m³.
- Post volume in concrete: 0.1 × 0.1 × 0.6 = 0.006 m³.
- Concrete per hole: 0.04241 − 0.006 = 0.03641 m³.
- Six holes: 6 × 0.03641 ≈ 0.2185 m³.
- With a 10% allowance: approximately 0.2403 m³, or 240 litres.
To estimate bags, divide the required litres by the finished concrete yield per bag printed on the product packaging and round up. For example, at an illustrative yield of 10 litres per bag, 240.3 litres requires 25 bags. Actual yields vary.
Before you buy
- Check whether the product is suitable for post fixing and follow its mixing or water-addition instructions.
- Check the specified post embedment and concrete-fill depth; they are not always the same.
- Check drainage, underground services, boundaries and any relevant manufacturer guidance.
- Allow for irregular holes and spillage, but don't use an arbitrary bag yield.
Frequently asked questions
Does a bigger post always mean more concrete?
Not if the hole size stays the same: a larger post occupies more of the hole. In practice, the required hole size may also change, so don't choose footing dimensions based on concrete savings.
Can I calculate rectangular holes?
Yes. Use hole length × width × concrete-fill depth, then subtract the portion occupied by the post.
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Note: Illustrative UK planning estimates only. Follow product instructions and obtain qualified advice for structural work, drainage or site-specific conditions.