Size rainwater harvesting tanks for single and multiple properties

Rainwater harvesting tank sizing

The Rainwater harvesting tank sizing tool provides methods for sizing rainwater harvesting tanks for stormwater management of runoff for individual and multiple properties. The method used is the same as that in the Rainwater harvesting code of practice BS 8515 ed. 2012 appendix A.


The ratio of annual Yield/Demand (referred to as Y/D) is used to evaluate the design requirements of a rainwater harvesting tank for effective stormwater management. The ‘Yield’ is estimated by multiplying the annual effective rainfall depth by the contributing roof area, while ‘Demand’ is estimated by multiplying the house occupancy by the per capita daily consumption of non-potable water. 

In the case of Y/D not exceeding 0.9 (which indicates there is normally space in the tank for storing rainfall runoff), the tank storage size and its likelihood of compliance (with specified stormwater control criteria) are calculated by the "Kellagher and Gerolin Methodology". If Y/D exceeds 0.9, the rainwater harvesting system is considered to be suitable for non-potable water supply and Interception storage only, but not for providing control of stormwater runoff. In this case, the the tank size is calculated based on the standard approach, which calculates the volume to be the lesser of 5% of the annual Yield (Y) or 5% of the annual Demand (D).

When to use this tool?

Use of rainwater harvesting meets sustainability objectives of saving and using water. It also is one of the main ways in which the Interception criteria is achieved where this is required for sites.

How to use the tool?

The tool is available as an online tool, which all registered users can access and use for free. 

What values will the tool give?

The Rainwater harvesting tank sizing tool provides tank sizing for single or communal property systems based on meeting stormwater control criteria.





HR Wallingford provides training in the use of UK SuDS tools, plus planning and design of SuDS to meet current standards and best practice. 

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