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Land Drain Calculator — How Much Gravel and Pipe Do I Need?
Calculate perforated pipe, gravel tonnage, and geotextile membrane for land drainage trenches. Pipe to BS 4962 / BS EN 1401; trench installation to BS EN 1610.
Total run length of land drain
600-900mm typical
300mm standard for mini-digger bucket
Strongly recommended to prevent drain silting up
Price per tonne of 20mm clean gravel
Extra pipe factor for junctions and connections (default 1.1 = 10% extra)
Depth of topsoil backfill above gravel zone (default 0.15m = 150mm)
Density of clean gravel in tonnes per m³ (default 1.6 for 20mm washed gravel)
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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). 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.
Frequently Asked Questions
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.
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.
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.
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. Use our Drainage Gradient Calculator to work out invert levels.
For a standard 300mm wide x 600mm deep trench with 100mm perforated pipe (allowing for 150mm topsoil backfill), the gravel-fill zone is 0.45m x 0.3m = 0.135 m³ per metre. After deducting the volume taken up by the pipe itself (a 100mm cylinder ≈ 0.008 m³/m), the calculator works to about 0.127 m³ of gravel per metre. At 1.6 tonnes per m³, that is roughly 0.20 tonnes (around 200kg) of 20mm clean gravel per metre of land drain. Order a little extra to allow for settling and compaction.
Related Calculators
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Calculate pipe bedding depth, haunch and pea gravel volume by trench width.
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Calculate foul drainage pipe routing, minimum cover depths and inspection chamber positions.
Surface Water Drainage Calculator
Calculate surface water pipe capacity from impermeable area and rainfall intensity.
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Last updated: February 2026
Verified against UK standards · estimates only, confirm with your supplier.