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How to size a Rainwater Harvester

System Sizing :

We’re all concerned about saving water, saving money not to mention saving the environment – not many would disagree that these are pressing needs.

These imperatives have resulted in a rapid increase in rainwater harvesting which is now a fast developing industry with various systems on the market. Indeed many mechanical and electrical consultants see rainwater and grey water harvesting as routine adjuncts to their product range.

Rainwater harvesting in particular is a viable alternative to using mains water in a range of non-potable (non-drinking) applications such as flushing toilets & urinals, process water, garden and other irrigation systems, laundry equipment, car washing and fire sprinkler systems – the list is almost endless. 

In an effort to ensure consistency the British Standards Institute (BSI) has published BS 515:2009 Rainwater Harvesting Systems – Code of practice (BS 8515:2009). This is because a thorough and detailed approach to rainwater harvesting is essential. Designing such a system requires looking at a number of variables such as flow rates, peak demand periods, yield and demand, and of course suitable filtration levels to maintain a clean and clear rainwater supply.

Not all of these factors are necessarily covered in BS 8515:2009, which defines the minimum standards required for rainwater harvesting systems but still serves as a useful guide.

There are a number of elements to be taken into consideration when sizing a rainwater harvesting system. To ensure optimum efficiency, it is important to strike a balance between the following factors:

•   Rainwater yield.
•   Rainwater demand.
•   Budget.
•   Overflow frequency.
•   Future proposed applications.


We don’t propose to get into all the intricacies of the formulae involved in calculating the yields and consequent size of the harvesting system you will need because you can click below to use a technical manual produced by Tricel – one of the leading rainwater harvesting system providers.

However to give you some idea of how this works here is an example with some typical assumptions built in:


Roof area (based on house 17m wide, 8m deep) :


square metres,

Annual rainfall in your area:



Total annual  collectable rain:




Typical daily usage of rain water indoors and in the garden:

Washing machine cycle needs 50 litres :


washing machine cycles per day

Toilets require 10 litres per flush:


flushes per day

Water needed in the garden etc:


litres per day

Total daily use:


litres per day



litres across a whole year

If total amount of rain collectable  is more than or about the same as this figure then you have a

coherent rain water "budget".


Assumed drought protection required of:



(This figure can be varied to see the effect on the size of tank required)

The optimal quantity of stored water:



Tank size calculator is now telling you that you need:

Use a 6500 litre tank.

To visit the Tricel Rainwater Harvester Technical Manual  CLICK HERE.

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