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

This calculator gives an indicative estimate of attenuation storage and crate numbers using the Rational single-storm difference method (inflow minus outflow over one design-storm duration). It is a starting point — see the note below on why a single duration is not the full picture.

The formulas

Peak inflow rate (l/s) = (Rainfall intensity × Impermeable area) / 3600

Storage volume (litres) = (Peak inflow - Allowable discharge) × Storm duration (seconds)

Number of crates = ceil(Storage volume / (Crate capacity × Void ratio))

How it works

The peak inflow rate is calculated from the design rainfall intensity falling on the impermeable (hard-surfaced) area. The allowable discharge rate (entered exactly as your LLFA specifies it) is subtracted to give the net excess flow that must be stored. That excess, multiplied by the storm duration, gives the storage volume.

The storage volume is then divided by the effective capacity of each crate (nominal capacity × void ratio, default 0.95) to determine how many crates are needed.

Why one storm duration is not enough

A single duration only samples one point. The critical storm — the one needing the most storage — is found by testing a range of durations: short storms are intense but brief, longer storms are gentler but deliver more total volume. The CIRIA C753 SuDS Manual and the susdrain attenuation method both require finding this maximum-storage (critical) duration. If your peak inflow at one intensity does not exceed the discharge, this method shows zero storage, but a longer, lower-intensity event may still require it — so test several durations here and take the largest result, or use full SuDS software.

Important notes

This is a simplified calculation suitable for initial sizing only. For planning applications, a full drainage strategy using software such as MicroDrainage / InfoDrainage or InfoWorks is typically required, modelling the full range of storm durations and return periods with FEH/FSR rainfall. The design should be carried out or verified by a drainage engineer.

Standards

Surface-water management should comply with Part H of the Building Regulations, the CIRIA SuDS Manual (C753), the National Standards for Sustainable Drainage Systems (Defra, 19 June 2025) and the Non-Statutory Technical Standards for Sustainable Drainage Systems (Defra, 2015). Climate change allowances follow the Environment Agency 'Flood risk assessments: climate change allowances' guidance (the flat 40% national uplift was withdrawn on 10 May 2022). Attenuation crates should hold BBA certification or equivalent.