StormFlow Calculation Form

Project Name


Postcode of Site

Catchment Area (Roof) m²

Area of Dense Surfacing (non-permeable) m²

Location of Tank

Expected loading if known

Storm Return Period

Allowance for Climate Change

Groundwater level

Soil Infiltration Rate (for a Soakaway Tank)


Allowable Discharge Rate (for an Attenuation Tank)

Additional information if required


Please note calculations are based on the information provided therefore it is important that all details are correct and accurate to ensure the most suitable solution is being recommended for your requirements.


Signed Name
DD slash MM slash YYYY

Catchment Area

This refers to the area of the roof from which rainwater is collected. It’s usually the horizontal projection of the roof surface. To find this, measure the length and width of your roof and multiply them together.

Area of Dense Surfacing (Non-Permeable)

This is the region where materials like concrete or asphalt prevent water from penetrating.Measure the length and width of these areas and multiply to get the total area in square metres.

Location of Tank

Specify where you plan to install the tank (e.g., in the garden, under a driveway). Consider accessibility for maintenance and the proximity to the area where the stormwater is collected.

Expected Loading if Known

This refers to the weight load the tank area is expected to support. This could be pedestrian traffic, vehicles, etc. Estimate based on the heaviest load likely to be encountered (e.g., 3 tonnes for light vehicles).

Storm Return Period

This indicates the statistical period of time, in years, in which the system capacity would be expected to be exceed once only. The chosen value depends upon the risk attached to the consequences of flooding and is often specified by the Planning Authorities when the scheme is being approved. Typical values for both soakaways and attenuation tanks are 1 in 30 or 1 in 100 years.

Allowance for Climate Change

This is an adjustment factor for potential changes in rainfall intensity and patterns due to climate change. This is often specified by the Planning Authorities at approval stage and is normally in the region of +40%.

Groundwater Level

The depth at which water is found below ground. For a soakaway to work effectively this level must be at least 1m below the base of the tank and for an attenuation tank it should ideally be below the base of the tank to avoid floatation issues.

Soil Infiltration Rate (for Soakaway Only)

This is a measure of how quickly water can soak into the soil. It can be determined through a percolation test or obtained from local soil surveys.

Click here to find out how to carry out a percolation test.

Allowable Discharge Rate (for Attenuation Tank Only)

This is the maximum rate at which water can be released from the tank into the drainage system. Local regulations often dictate this to prevent overloading public sewer systems.