Post Hole Concrete Calculator
Work out how much concrete you need for round post holes — per hole and for the whole fence.
Your result
Enter your hole size above to see how much concrete you need.
Hole size, post embedment depth and footing requirements depend on the post, the fence height, wind loading and your soil. For retaining structures, tall fences or anything carrying load, follow the manufacturer's specification or your engineer's design.
How post hole concrete is calculated
A post hole is a cylinder, so the volume comes from the area of the circle multiplied by the depth:
Volume (m³) = π × (diameter ÷ 2)² × depth
For a 300 mm diameter hole 600 mm deep: π × 0.15² × 0.6 = 0.042 m³ per hole. Ten of those is 0.42 m³ before any allowance.
Note how sharply diameter drives the result — it is squared. Going from a 300 mm hole to a 400 mm hole does not add a third to your concrete, it adds nearly four fifths. If you are hiring an auger, the bit size you choose has more effect on your order than the depth does.
Subtracting the post
The post occupies part of the hole, so strictly you need slightly less concrete than the full cylinder. Enter a post width above and the calculator subtracts it, assuming the post sits on the bottom of the hole.
For a 90 × 90 mm post in a 300 mm hole the difference is around 12% — worth accounting for across a long fence, negligible for two or three posts. Many people ignore it deliberately and treat it as extra allowance.
Bags or ready-mix for post holes?
Almost always bags. Post holes are spread out along a fence line, they are poured one at a time as each post is plumbed and braced, and the total volume is usually well under half a cubic metre. A concrete truck cannot wait around while you level twenty posts.
The exception is a long boundary fence with large holes — thirty 400 mm holes at 900 mm deep is over 3 m³, at which point a delivery into barrows, or a mixer on site, starts to make sense.
Rapid-set post mixes are a common alternative for domestic fencing. Some are poured dry into the hole with water added afterwards, in which case the bag's stated coverage — often given as holes per bag rather than litres — is what to follow.
Common post hole sizes
| Job | Often around | Concrete per hole |
|---|---|---|
| Colorbond / paling fence post | 300 mm dia × 600 mm | ~0.042 m³ |
| Taller or exposed fence post | 300 mm dia × 900 mm | ~0.064 m³ |
| Gate post | 400 mm dia × 900 mm | ~0.113 m³ |
| Pergola / verandah post | 400 mm dia × 600 mm | ~0.075 m³ |
General examples only. Gate posts in particular carry ongoing load from the gate swinging and are usually dug deeper and wider than the run of the fence.