How deep should a land drain trench be?

A typical land drain trench should be 600mm to 900mm deep and 300mm wide. For garden drainage, 600mm is usually sufficient. For agricultural land and sports pitches, 750-900mm is recommended. The pipe should be laid on a minimum 100mm gravel bed and covered with at least 50mm of gravel above the pipe crown.

What type of gravel should I use for land drains?

Use 20mm clean (washed) gravel or pea shingle for land drains. The gravel must be single-sized (not a graded mix) and free from fines (dust and clay), which would clog the perforations in the pipe. Do not use building aggregate, sharp sand, or crusher run as these contain fines that will block the drain over time.

Do I need geotextile membrane for a land drain?

Yes, geotextile membrane (also called filter fabric) is strongly recommended. It wraps around the gravel fill to prevent fine soil particles from migrating into and clogging the gravel and pipe perforations. Without it, the drain will gradually silt up and lose effectiveness. Use a non-woven geotextile with a suitable filtration grade for your soil type.

What fall should a land drain have?

Land drains should have a minimum fall of 1:200 (0.5%) but ideally 1:100 (1%) or steeper. This is 5mm per metre at minimum, or 10mm per metre ideally. The fall ensures water flows through the perforated pipe to the outlet point (soakaway, ditch, or watercourse). Always check falls with a laser level or spirit level on a straight-edge during installation.

How much gravel do I need per metre of land drain?

For a standard 300mm wide x 600mm deep trench with 100mm perforated pipe, you need approximately 0.135 m³ of gravel per metre of trench (allowing for 150mm topsoil backfill). At 1.6 tonnes per m³, this equates to roughly 0.22 tonnes (220kg) of 20mm clean gravel per metre of land drain.

Land Drain Calculator — How Much Gravel and Pipe Do I Need?

Calculate perforated pipe, gravel tonnage, and geotextile membrane for land drainage trenches. Based on BS 8301 land drainage standards.

Trench Length

Total run length of land drain

Trench Depth

600-900mm typical

Trench Width

300mm standard for mini-digger bucket

Pipe Diameter

100mm is standard for most land drains

Gravel Bed Depth

Minimum 100mm gravel below pipe

Include geotextile membrane

Strongly recommended to prevent drain silting up

Gravel Price Per Tonne (optional)

Price per tonne of 20mm clean gravel

How We Calculate This

This calculator determines the materials needed for a land drainage system based on your trench dimensions, pipe diameter, and gravel requirements.

The formulas

Pipe length = Trench length × 1.1 (10% extra for junctions and connections)

Gravel volume = (Trench length × Gravel zone depth × Trench width) - Pipe volume

The gravel zone depth is the trench depth minus 150mm topsoil backfill at the surface. The pipe volume (a cylinder) is subtracted to give the actual gravel fill volume.

Gravel tonnage

Gravel volume is converted to tonnes using a density of 1.6 tonnes per m³, which is the standard figure for 20mm clean (washed) gravel. Always order slightly more than calculated to account for settling and compaction.

Geotextile membrane

Membrane area is calculated as the trench length multiplied by the developed width (trench bottom + two sides + 300mm overlap each side). This ensures the membrane fully wraps the gravel fill with sufficient overlap at the top to fold over.

Standards

Land drainage should comply with BS 8301 (Code of Practice for Building Drainage) and CIRIA Report C753 for sustainable drainage. Perforated pipes should comply with BS EN 1401 for PVC or BS 4962 for flexible corrugated pipes.

Frequently Asked Questions

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Last updated: February 2026

All calculations are estimates. Verify with your supplier.