What this calculator does
Fence post hole depth works out how deep to set a fence or deck post below grade, and the total post length to buy. 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
FormulaBuried depth = above-ground height × burial ratio (commonly 1/3 to 2/5), deepened if needed to clear the frost line, plus a gravel drainage base
The inputs explained
| Field | What to enter |
|---|---|
| Height above ground (finished) (m) | A number, measured in m. Starts at 1.8. |
| Burial ratio | A number. Starts at 0.33. |
| Gravel base below the post (mm) | A number, measured in mm. Starts at 100. |
| Local frost depth (0 if none) (mm) | A number, measured in mm. Starts at 0. |
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 height above ground (finished)
Every other input is held at the calculator’s starting values while height above ground (finished) varies. Select any row to load that scenario into the calculator.