Storm Water Attenuation Calculator — How Much Storage Do I Need?

Storm water attenuation is the temporary storage of rainwater during heavy storms to prevent flooding downstream. When a site is developed (roofs, driveways, patios), rainwater runs off much faster than from the original greenfield. Attenuation tanks or crates store the excess water and release it slowly at a controlled rate (the greenfield runoff rate), preventing overwhelm of the drainage system or downstream flooding.

Design rainfall is location-, return-period- and duration-specific — there is no single national 'typical' figure (the 50 mm/hr default in this tool is illustrative only). Derive your design rainfall from FEH/FSR depth-duration-frequency data for your site (e.g. uksuds.com or the FEH web service). On climate change: the Environment Agency withdrew the flat 40% national peak-rainfall uplift on 10 May 2022. It is now replaced by management-catchment-specific allowances split by epoch (2050s / 2070s) and percentile (central 50th / upper-end 95th) — look up your value in the EA 'Flood risk assessments: climate change allowances' peak-rainfall dataset. Under the National Standards for Sustainable Drainage Systems (Defra, 19 June 2025), major developments must use the upper-end allowance. Your LLFA or SuDS Approving Body will confirm the return period and allowance for your planning condition.

The greenfield runoff rate is the rate at which water would naturally drain from a site before development, set by soil type, topography and catchment area. Greenfield rates are commonly in the order of 2-8 l/s per hectare and are estimated using methods such as IH124 / QBAR (see uksuds.com). The Defra Non-Statutory Technical Standards apply a floor of 2.0 l/s per hectare where the calculated rate is lower — note that is a rate per hectare, not an absolute 2 l/s. Some LLFAs instead floor at 1 l/s absolute, or at the 1-in-1-year greenfield rate. Enter the discharge figure (in l/s) your LLFA / SuDS Approving Body has specified; this calculator uses it exactly as entered and does not impose its own floor.

Attenuation crates (also called soakaway crates, geocellular storage, or stormwater crates) are modular plastic units that are assembled underground to create a void space for storing water. They have a void ratio of approximately 95%, meaning 95% of the crate volume is available for water storage. Common brands include Polypipe, Wavin, and Graf. They are wrapped in geotextile membrane and buried in an excavation. Use our Soakaway Crate Calculator to size a crate system for your project.

Attenuation systems themselves do not usually require separate planning permission, but they are typically a condition of planning approval for new developments. Under the National Planning Policy Framework (NPPF) and local planning policies, developments must demonstrate that surface water runoff will not increase flood risk downstream. A surface water drainage strategy is usually required as part of the planning application.