How big should a rain garden be?

A rain garden should be 2-5% of the catchment area it serves. For a 100 m² roof, a 3-5 m² rain garden is typical. On clay soils with low infiltration, aim for 5%. On sandy soils, 2% may suffice. The garden should be at least 600mm deep with a ponding depth of 100-200mm above the filter media.

What soil mix do I need for a rain garden?

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.

How does a rain garden work?

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.

What plants work in a UK rain garden?

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.

Does a rain garden need an overflow?

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).

TradeCalculator

Rain Garden Calculator — Filter Media, Ponding & Overflow

Calculate rain garden area, filter media volume, ponding depth, mulch and overflow for SuDS-compliant planting beds.

Catchment Area

Roof/paving area draining to rain garden

Garden as % of Catchment

2-5% typical (3% standard)

Filter Media Mix

Filter Media Price

Mulch Price

How We Calculate This

This calculator determines rain garden sizing, filter media volume, mulch and overflow requirements based on your catchment area.

Sizing

Garden Area = Catchment Area x Percentage (2-5%)
Filter Media = Garden Area x Soil Depth (600mm)
Ponding Volume = Garden Area x Ponding Depth (150mm)

Storage capacity

Total storage = Ponding volume + (Filter media volume x 30% void ratio). The 30% void ratio accounts for the pore space in the engineered soil that stores water temporarily.

Frequently Asked Questions

Related Calculators

Topsoil Calculator

Calculate how many tonnes or bulk bags of topsoil you need.

Gravel Calculator

Work out gravel or aggregate quantities in tonnes and bulk bags.

Type 1 Sub-Base Calculator

Calculate Type 1 MOT sub-base quantities for driveways and paths.

Soakaway Sizing Calculator

Size a soakaway to UK BRE DG 365 standards for your roof area.

Fence Post Spacing Calculator

Calculate fence post quantities and optimal spacing.

Last updated: March 2026

All calculations are estimates. Verify with your supplier.