To get an approximate calculation of how much rainwater can be harvested annually from a building, all you need to do is measure the length and width of the building to find the area in square metres, then multiply this by the annual local rainfall in cubic metres.

You can divide it by 12 to find the monthly rainwater harvesting potential

Use can also use our simple rainwater harvesting calculator which will also suggest the ideal tank size to maximise how much rainwater you can collect

Example Calculation:

Length 5m x Width 4m = 20m2

20 x Average annual rainfall 0.9 (900m/annum) = 18m cubed of water per year

1 cubic metre of water = 1000L

Hence, 18 cubic metres of water = 18000L of harvested rainwater

Rainfall varies depending on your location, which can be checked by county throughout the UK.

A filter is essential to prevent leaves, sediment, grit and other debris from entering the tank and causing problems with pumping equipment and water quality. We always recommend fitting a rainwater filter to filter out debris and prevent it from getting into the tank.

Additionally, matching the roof area to the correct filter is critical to ensure your system runs smoothly.

There are specific rainwater harvesting tanks (or non-potable tanks) designed for harvesting rainwater. However, a potable (drinking water) tank can be also used for the same purpose. Although, the opposite is not true; a rainwater tank cannot be used to store drinking water.

To learn more about the difference between potable and non-potable tanks, please read our blog on potable vs non-potable water tanks.

Several features help keep rainwater fresh and high-quality:

Filter: Prevents sediment and debris from entering the system. Choosing a filter suited to your roof size is essential.

Calmed inlet: Minimises stirring in the tank and adds oxygen at the base, helping the water stay clear.

Surface debris removal: Lighter particles like pollen form a floating layer, which is removed via an overflow siphon to keep the water clean.

Floating intake: Water is drawn from near the top of the tank, where it’s typically of the highest quality.

Underground storage: Tanks stored below ground benefit from cool, dark conditions (usually 6–8°C), which prevent algae and microorganisms from developing and maintain high water quality year-round.

For all rainwater harvesting systems, a mains backup can be installed. This features a probe that determines how full the tank is. It automatically switches on a mains water supply when the tank's water level is running low.

For rainwater harvesting systems with pump equipment, an annual tank service, health-check and professional clean is recommended to ensure all components are free of debris and functioning well.

Gutters should be cleaned once a year. It's also important to check the filter at least every six months to keep it clear; modern filters are largely self-cleaning and require minimal upkeep.

We offer a full rainwater harvesting supply and installation/fitting service to customers in the South West. The areas we cover include:

- Somerset

- Dorset

- Devon

- Wiltshire

- Bournemouth

- Poole

We recommend that rainwater tanks are cleaned and serviced once a year. We offer a full water tank clean and health check service to customers across the South West.

Learn more about how often to clean rainwater tanks on our blog!