How many soakaway or attenuation crates do I need?

The basic calculation

A geocellular tank stores water in the void inside the modules. The number of modules is therefore:

modules = required storage volume ÷ (module volume × void ratio)

For a 1000 × 500 × 400 mm module, the gross volume is 0.2 m³. At a 95% void ratio, each module stores about 0.19 m³, so 10 m³ of storage needs about 53 modules. The calculator above does this for common module sizes and passes the result into a quotation request.

Where the required volume comes from

The storage volume is the output of the drainage design, not an input you choose. For attenuation it depends on the site area, the design storm, the allowable discharge rate and climate-change allowances; for soakaways it also depends on the measured infiltration rate of the soil. In the UK, soakaways are commonly sized using BRE Digest 365 infiltration testing, and SuDS design follows the national standards referenced in planning policy. If you only have an area and a rainfall depth, the "catchment" mode gives a rough first estimate — treat it as budgeting only.

Things that change the module count

• Void ratio — most modules are around 95%, but check the product data.
• Layers and footprint — the same volume can be a wide single layer or a deeper multi-layer tank; the load class and cover depth limit the number of layers.
• Inspection and access modules — these take up positions in the layout.
• Freeboard and silt allowance — designers often add margin for sediment.

Before you order

Confirm the load class for the traffic above the tank and the minimum/maximum cover depth, decide on geotextile or geomembrane wrap, and check whether your authority requires a specific product approval.