Rain Garden Calculator — Filter Media, Ponding & Overflow

The UK standard is around 20% of the catchment area it serves. The RHS advises multiplying the roof area by 0.2 (20%), and the gov.uk National Standards for SuDS (2025) deem an unlined bioretention feature compliant where the impermeable area is up to 5 times the vegetated area — i.e. at least catchment ÷ 5 (20%). So a 100 m² roof needs roughly a 20 m² rain garden. On clay or low-infiltration soils, use the upper end (20%+); free-draining sandy soils can sometimes work nearer 10-15%. Aim for around 600mm of filter media with a ponding depth of 100-200mm above it.

The standard UK rain garden filter media is 60% sharp sand, 20% compost and 20% topsoil. This provides good infiltration (typically 100-300 mm/hr) while retaining enough moisture and nutrients for planting. On heavy clay sites, increase sand to 70%. The media should be 600mm deep over a 150mm gravel drainage layer.

Rainwater from roofs, driveways or paths flows into the shallow planted depression. Water ponds temporarily (draining within 24-48 hours), filtering through the engineered soil media which removes pollutants. Plants take up nutrients and water. Excess water drains through a gravel layer to an overflow pipe connected to the drainage system or soakaway.

Choose plants that tolerate both wet and dry conditions. Good UK choices include: purple loosestrife, marsh marigold, yellow flag iris, sedges (Carex species), rushes, astilbe, heuchera, and ornamental grasses. Avoid invasive species. The RHS and CIRIA SuDS Manual list suitable species for different soil types and aspects.

Yes, every rain garden needs an overflow pipe to handle storms that exceed the ponding capacity. A 100mm diameter pipe set at the maximum ponding level connects to the existing drainage system or a soakaway. Without an overflow, extreme rainfall could cause uncontrolled flooding. Also include a pre-treatment inlet (gravel strip or silt trap).