What stone should I use for a French drain?

Use clean, washed stone with no fines (dist) typically 20 mm (¾ in) or 40 mm (1.5 in) in size. Do not use road-base or "crush and run" as the fines will clog the pipe perforations.

Is a French drain pipe installed holes-up or holes-down?

Holes should always face DOWN. Water enters the drain by rising into the pipe from the bottom of the trench once the water table reaches that level.

Why do I need geotextile fabric?

Non-woven geotextile fabric (filter fabric) prevents surrounding soil from migrating into the drainage stone and eventually clogging the pipe, ensuring the drain works for years.

Drainage & Earthworks

French Drain Calculator

Design and estimate your drainage trench. Calculate the volume of drainage stone needed after accounting for pipe displacement, find the required filter fabric area, and get a total material list for your project.

French Drain Calculator

Estimate drain rock, excavation, fabric, pipe length, and slope drop

Drain length

Trench width

Trench depth

Gravel fill depth

Pipe diameter (optional)

Waste allowance

Fabric overlap (optional)

Slope (optional)

Results

Enter dimensions above to calculate

Anatomy of a French Drain

A proper French drain uses perforated pipe at the bottom, surrounded by clean stone, and wrapped entirely in a permeable geotextile envelope.

Step-by-Step Formulas

1. Trench Volume

Volume = Length × Width × Depth

Measure the length of the drain run and the cross-section of the trench. Trench width is typically 300–450 mm (12–18 in) depending on soil and pipe size.

2. Pipe Displacement

Pipe Volume = π × (Radius)² × Length

The drainage pipe (usually 100 mm / 4 in perforated pipe) occupies space inside the trench. We subtract its volume from the pipe-bedding zone to avoid over-ordering stone.

3. Drainage Aggregate

Stone Volume = Trench Volume − Pipe Volume

The void space around the pipe is filled with clean, washed 20 mm (¾ in) drainage stone. This is your net order volume.

4. Filter Fabric (Geotextile)

Fabric Area = Length × (Perimeter + Overlap)

To prevent sediment from clogging the stone, the entire trench should be wrapped in non-woven geotextile fabric. Include a 300 mm (12 in) overlap at the top.

Worked Example

Run: 15 m long, 40 cm wide, 60 cm deep. 100 mm (4") perforated pipe. 10% stone waste.

Gross Trench Volume = 15 × 0.4 × 0.6 = 3.60 m³

Pipe displacement = π × 0.05² × 15 ≈ 0.12 m³

Net Stone Volume = 3.60 − 0.12 = 3.48 m³

Order Stone (+10% waste) = 3.48 × 1.10 = 3.83 m³

Geotextile (Perimeter + 0.3m overlap) = 15 × (2.0 + 0.3) = 34.5 m²

💡 Pro tip

Always use non-woven geotextile rather than woven weed mat. Non-woven fabric allows much faster water flow while still trapping soil fines. Also, ensure your drain outlet (daylight or dry well) is free and clear to avoid water backing up.

Frequently Asked Questions

What stone should I use for a French drain?: Use clean, washed stone (no fines) typically 20 mm (¾ in) or 40 mm (1.5 in). Avoid road-base or "crush and run" aggregates.
Is a French drain pipe installed holes-up or holes-down?: Holes must face DOWN. Groundwater rises into the pipe from the bottom as the water table reaches the trench level.
Why do I need geotextile fabric?: It prevents soil fines from migrating into and clogging the drainage stone, ensuring long-term drainage performance.