Post Hole Concrete Calculator
Work out the concrete needed to set fence or deck posts: the volume per hole after the post is in, the total, and the number of bags.
Bags of concrete for your fence posts.
How the post hole concrete calculator works
A post hole is a cylinder; the post takes up a square prism inside it. Concrete per hole = (π × (diameter ÷ 2)² − post width²) × depth. Multiply by the number of holes and divide by the yield of your bag — about 10 litres from a 20 kg bag, 0.6 cubic feet from an 80 lb bag.
The usual rule is a hole three times the post width and a third of the post's total length deep, with the post sitting on a few centimetres of gravel for drainage.
Formula: per hole = (π (d ÷ 2)² − p²) × depth
Worked examples
| Inputs | Bags to buy | Note |
|---|---|---|
| 10 holes, 300 mm × 600 mm, 100 mm posts | 37 | 36.4 L each — 37 bags of 20 kg |
| 10 holes, 250 mm × 600 mm, 90 mm posts | 25 | a smaller auger |
| 12 holes, 12 in × 24 in, 4 in posts | 27 | 80 lb bags |
FAQFrequently asked questions
How deep should a fence post hole be?
A common rule is a third of the post's total length — 600 mm for a 1.8 m fence — and below the frost line where frost heave occurs. Local codes may set minimums.
How wide should the hole be?
About three times the post width: 300 mm for a 100 mm post. Wider holes use a lot more concrete for little gain.
Do I need concrete at all?
For gates, corners and ends, yes. Intermediate posts in firm ground can be set in compacted gravel, which also drains better.
Should the post sit on concrete or gravel?
On 50–100 mm of gravel so water drains away from the end grain, with concrete around the sides and sloped away at the top.
Where these figures come from
- The Engineering ToolBox — material densities, coverage and unit data — bulk densities, concrete at 2,400 kg/m³, board and sheet sizes
- Cement Concrete & Aggregates Australia — concrete basics and yields — bag yields, density 2,400 kg/m³
- Australian Building Codes Board — the National Construction Code
Last checked: September 2026. Coverage rates and material sizes are typical manufacturer figures — check the product you buy.