Drainage & Water Management Calculators

Drainage is a critical element of every UK building project, governed primarily by Approved Document H (Drainage and Waste Disposal) of the Building Regulations. Whether you are installing foul drainage for a new-build house, laying land drains to manage waterlogged ground, or designing a storm water attenuation system for a planning condition, getting the calculations right is essential to avoid costly rework and Building Control rejections.

For properties not connected to mains sewerage, off-mains drainage solutions such as septic tanks and sewage treatment plants must comply with the General Binding Rules (England) 2020 and BS EN 12566. Since 1 January 2020, septic tanks can no longer discharge directly to a watercourse in England — a drainage field or sewage treatment plant is required instead. Our septic tank sizing calculator uses the BS 6297 formula (C = 2000 + 180P) to determine the minimum tank capacity, while our sewage treatment plant calculator sizes package treatment plants by population equivalent.

Surface water drainage is increasingly regulated through SuDS (Sustainable Drainage Systems) requirements. Planning authorities routinely require that post-development runoff rates match pre-development (greenfield) rates, often necessitating attenuation storage. The storm water attenuation calculator helps you size storage volumes using peak inflow rates and allowable discharge rates in accordance with the CIRIA SuDS Manual (C753) and Part H.

Before installing any soakaway or drainage field, a percolation test must be carried out in accordance with BRE Digest 365. The test determines the percolation rate (Vp value) of the soil, which in turn determines whether a soakaway is feasible and what size drainage field is required. A Vp value between 15 and 100 seconds per millimetre indicates suitable ground for soakaway drainage; outside this range, alternative solutions must be considered.

Inspection chambers and manholes are required at changes of direction, gradient, or pipe size, and at junctions in the drainage run. Our manhole calculator helps determine the number of precast rings or bricks needed based on depth, along with cover and frame sizing. For surface water drainage channels, ACO-style channel drains are widely used across driveways, patios, and commercial premises, with components conforming to BS EN 1433.

Land drainage is essential for waterlogged sites, sports pitches, and agricultural land. A typical land drain system consists of perforated pipe laid in a gravel-filled trench with geotextile membrane wrapping, as specified in BS 8301 (Code of Practice for Building Drainage). Correct calculation of gravel volumes and membrane areas avoids material shortages and ensures effective drainage performance for years to come.