Land Drainage Calculator — Pipe Layout, Gravel & Trenching

For garden, lawn and sports-turf drainage the spacing in this tool is sized so no point is more than about 2.5m from a drain — heavy clay 5m, clay 7m, loam 10m, sandy loam 15m (closer spacing for heavier, less permeable soils). These are intensive-drainage figures (pavingexpert garden guidance). Agricultural field drains are spaced much wider — typically 20m or more (AHDB) — so for an arable field use AHDB figures, not these. Always test soil permeability before designing a system.

A herringbone pattern has a main collector pipe running down the centre (or along one edge) with lateral drains branching diagonally at roughly 45 degrees from both sides, like a fish skeleton. For a given drain spacing the total length of lateral pipe is the same whether the laterals run diagonally (herringbone), straight across (parallel/gridiron) or fan out — angled laterals are individually longer, but you need proportionally fewer of them, so the totals cancel out (total drain length = area ÷ spacing). This calculator therefore sizes the pipe from the area and spacing; the layout choice describes the physical arrangement, not the quantity. The collector should run in the direction of the natural fall to the outfall.

Land drains are typically installed at 600-900mm depth. Minimum depth is 450mm to avoid damage from cultivation or foot traffic. The collector pipe should be deeper than the laterals to ensure water flows by gravity. Lay the system to a gentle fall in the region of 1:100 to 1:200 towards the outfall (pavingexpert); 1:200 is 5mm fall per metre — use our Drainage Fall Calculator for gradients. Check the outfall level first — this determines maximum drain depth.

Use clean, washed 10-20mm gravel with at least 75mm around and above the perforated pipe (pavingexpert). Use our Gravel Calculator for quantities. Fill the trench to roughly half depth with gravel. Best practice is to line the trench with geotextile fabric first and wrap it over the top of the gravel (a “taco” wrap) so the membrane encapsulates the gravel zone and stops soil washing in — this calculator sizes the fabric for the gravel zone plus a top overlap, not the full trench depth. Backfill above with topsoil. Gravel weight assumes a typical 1.6 t/m³ loose density (UK range ~1.5-1.7 t/m³; confirm with your supplier).

The collector pipe is typically 80-100mm diameter perforated twinwall pipe for domestic/garden use. For larger areas or commercial land drainage, use 100-150mm. The collector must be larger than the laterals (usually 60-80mm). Use solid (non-perforated) pipe for the final run to the outfall to prevent water re-entering the pipe.