Land Drain Calculator
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Land Drain Calculator
Work out the perforated pipe, clean stone (gravel) tonnage and geotextile membrane for a land-drainage or French-drain trench — plus a layout helper to estimate total trench length from a field area and drain spacing. Design to BS EN 752:2017 and land-drainage good practice.
Trench materials
Enter the trench and pipe — the tool removes the pipe volume from the gravel using exact geometry and works out the membrane to line (or wrap) the trench.
Spacing & total trench length
Estimate the total length of lateral drains for an area from the drain spacing, which depends on the soil. Add a main/collector run to carry it to the outfall.
Spacing is indicative — the right figure depends on soil permeability, the depth of the drains and how wet the site is. A field permeability/percolation test gives the reliable value. The total length here can be carried straight into the trench take-off tab.
Reference & rules
Typical materials, spacing and good-practice points for land drainage.
| Soil | Permeability | Spacing |
|---|---|---|
| Clay / heavy | low | ~5–10 m |
| Loam / medium | moderate | ~10–20 m |
| Sandy / light | high | ~20–40 m |
Materials. Perforated/slotted land-drain pipe (often 80–160 mm), surrounded by clean angular stone (typically 20–40 mm, ≈1.5 t/m³). Wrap the stone in a non-woven geotextile to stop fines clogging the drain. A blinding/topsoil cap is added where the drain runs under grass or planting.
Falls & outfall. Lay land drains to a steady fall (commonly 1:100 to 1:200) towards a positive outfall — a ditch, watercourse or soakaway. A discharge to a watercourse may need consent; never connect a land drain to a foul sewer.
Standards. Drainage outside buildings is covered by BS EN 752:2017 (this superseded the withdrawn BS 8301). Agricultural field drainage also draws on long-standing land-drainage practice (e.g. former MAFF/ADAS guidance).
This land drain calculator works out the perforated pipe, clean stone (gravel) tonnage and geotextile membrane for a land-drainage or French-drain trench, plus a layout helper to estimate the total trench length from a field area and drain spacing. The design follows BS EN 752:2017 and land-drainage good practice. Where a land drain outfalls to a ditch or stream you may also need ordinary watercourse consent.
Trench materials take-off
Enter the trench length, width and depth, the perforated pipe diameter and how the trench is lined, and the calculator removes the pipe volume from the gravel using exact geometry and works out the membrane needed to line or wrap the trench. You can also model a stone-only French drain with no pipe.
Spacing and total trench length
The layout helper estimates the total trench length from a field area and a chosen drain spacing, so you can size the job before you set out. Spacing depends on the soil — heavier, less permeable ground needs drains closer together.
Where the water goes
A land drain has to discharge somewhere lawful — usually a ditch or watercourse, which can need consent, or a soakaway where the ground allows. If you are draining foul effluent rather than groundwater, that is a drainage field, not a land drain, and is sized differently.
Further reading: ordinary watercourse consent explained · source control: SuDS at plot level · gov.uk SuDS technical standards.
Frequently asked questions
A land drain is a perforated pipe that collects groundwater; a French drain is a stone-filled trench that does the same, sometimes without a pipe. The calculator handles both.
It depends on the trench size and pipe diameter. The calculator removes the pipe volume from the trench and gives the stone volume and tonnage.
Closer on heavy clay, wider on free-draining soil. The layout helper lets you set the spacing and gives the total trench length.
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