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

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). The two side heights run only up the gravel-fill zone — i.e. the trench depth minus the topsoil backfill cap, not the full trench depth — because the topsoil layer above the gravel is not wrapped. So for the default 600mm trench with 150mm topsoil, each side is taken as 450mm. This ensures the membrane fully wraps the gravel fill with sufficient overlap at the top to fold over before the topsoil is replaced.

Standards

Perforated subsoil/field drains should comply with BS 4962 (plastics pipes for subsoil field drains) or BS EN 1401 (PVC-U non-pressure underground drainage). Trench excavation, pipe bedding and backfill should follow BS EN 1610 (construction and testing of drains and sewers). Where the land drain discharges surface water, the outfall and disposal route should meet Approved Document H (drainage and waste disposal); for sustainable drainage design see the CIRIA SuDS Manual (C753).

Note: BS 8301 (Code of Practice for Building Drainage) was withdrawn in 1998 and replaced by the BS EN 752 series (current edition BS EN 752:2017, drain and sewer systems outside buildings) — do not specify to BS 8301.