What this calculator does
French drain materials works out gravel and pipe needed for a French drain trench. 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
FormulaGravel volume = trench volume − pipe volume; pipe and drainage fabric run the trench length plus an allowance for fittings
The inputs explained
| Field | What to enter |
|---|---|
| Trench length (m) | A number, measured in m. Starts at 15. |
| Trench width (m) | A number, measured in m. Starts at 0.3. |
| Trench depth (m) | A number, measured in m. Starts at 0.45. |
| Perforated pipe diameter (mm) | A number, measured in mm. Starts at 100. |
| Gravel price ($/m³) | A number, measured in your currency/m³. Starts at 65. |
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 trench length
Every other input is held at the calculator’s starting values while trench length varies. Select any row to load that scenario into the calculator.
How the answer changes with trench length
| Trench length (m) | Gravel needed | Pipe length | Trench volume |
|---|---|---|---|
| 7.5 | 0.95 m³ | 8.3 m (10% extra for fittings) | 1.01 m³ |
| 11.25 | 1.43 m³ | 12.4 m (10% extra for fittings) | 1.52 m³ |
| 15 | 1.91 m³ | 16.5 m (10% extra for fittings) | 2.03 m³ |
| 22.5 | 2.86 m³ | 24.8 m (10% extra for fittings) | 3.04 m³ |
| 30 | 3.81 m³ | 33.0 m (10% extra for fittings) | 4.05 m³ |
| 45 | 5.72 m³ | 49.5 m (10% extra for fittings) | 6.08 m³ |