What this calculator does
Gravel is the trench volume less what the pipe occupies. A 15 metre trench 300 mm wide and 450 mm deep holds 2.03 cubic metres, of which the pipe takes 0.12, leaving 1.91 cubic metres of gravel.
Pipe and fabric are both calculated at 10% over the trench length, which covers fittings, overlaps and the inevitable mistake. The fabric matters more than people expect: without it, fine soil migrates into the gravel over a few years and the drain silts up and stops working.
The formula
The trench volume is length times width times depth, and the pipe volume is subtracted using its diameter. Pipe and fabric lengths add 10% to the trench length for fittings and overlap. The calculation assumes a uniform rectangular trench, so a drain that deepens along its fall needs the average depth entered.
| Term | Meaning |
|---|---|
| French drain | A gravel-filled trench with a perforated pipe, carrying water away from an area. |
| Perforated pipe | Slotted pipe laid in the gravel, usually with the holes facing down. |
| Drainage fabric | Geotextile wrapping the gravel, stopping soil migrating in and silting the drain. |
| Fall | The slope along the trench, typically 1 in 100 minimum, which moves the water. |
The inputs explained
| Field | What to enter |
|---|---|
| Trench length (m) | Trench length in metres. |
| Trench width (m) | Trench width. 300 mm is common for a domestic drain. |
| Trench depth (m) | Trench depth. Use the average if the trench deepens along its fall. |
| Perforated pipe diameter (mm) | Perforated pipe diameter in millimetres. 100 mm is standard. |
| Gravel price ($/m³) | Gravel price per cubic metre. |
When to use it
Draining a wet garden
The classic domestic application, intercepting water before it reaches a problem area.
Relieving water behind a retaining wall
Hydrostatic pressure is what fails retaining walls, and a drain behind them relieves it.
Protecting a foundation
A perimeter drain keeps water away from footings and basement walls.
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 much gravel for each trench depth?
The same trench at a range of depths.
| Trench depth | Gravel needed | Trench volume | Estimated gravel cost |
|---|---|---|---|
| 300 mm | 1.23 m³ | 1.35 m³ | $80.09 |
| 450 mm | 1.91 m³ | 2.03 m³ | $123.97 |
| 600 mm | 2.58 m³ | 2.70 m³ | $167.84 |
Questions
Which way up do the pipe holes go?
Downward, which surprises people. Water rises into the pipe from the gravel bed below, and holes facing down let the pipe fill from the lowest point and drain completely. Holes facing up mean water must rise to the pipe crown before entering.
Do I need drainage fabric?
Yes, in almost all cases. Without it, fine soil washes into the gravel and progressively blocks the voids, and the drain silts up within a few years. The fabric is a small part of the cost and it is what determines whether the drain still works in a decade.
What fall does a French drain need?
At least 1 in 100, so 10 mm of drop per metre, and more is better. Without fall the water sits in the trench rather than going anywhere. The drain also needs somewhere to discharge to, which is often the harder part of the design.
What gravel should I use?
Clean, washed, angular aggregate around 20 mm, with no fines. Fines are exactly what clogs a drain. Rounded pea gravel works but packs tighter and holds less water than angular stone of the same size.
For gravel quantities generally, see the gravel calculator. For retaining walls that need drainage, see the retaining wall block calculator.