What this calculator does
Post hole concrete works out concrete needed to set posts in round holes, allowing for the post itself. Enter your own figures above and the answer updates as you type: nothing is fixed in the code, so the result reflects exactly the numbers you supply.
The formula this calculator evaluates is printed under the tool and explained below, so you can check the working by hand or reuse it in a spreadsheet.
The formula
FormulaVolume per post = π×(hole radius)²×depth − (post cross-section)×depth
The inputs explained
| Field | What to enter |
|---|---|
| Hole diameter (mm) | A number, measured in mm. Starts at 300. |
| Hole depth (mm) | A number, measured in mm. Starts at 600. |
| Post width (square post) (mm) | A number, measured in mm. Starts at 100. |
| Number of posts | A number. Starts at 6. |
| Price per 20 kg bag ($) | A number, measured in your currency. Starts at 9. |
Worked examples
Every figure in the tables below is produced by this page’s own calculator at build time, so the numbers and the tool always agree. Select any row to load that scenario.
How the answer changes with hole diameter
Every other input is held at the calculator’s starting values while hole diameter varies. Select any row to load that scenario into the calculator.