What Size Rainwater Tank Do I Need?

As a simple rule of thumb, for domestic system you will require about 1000 litres of storage per person.

There are more detailed calculations that can be performed to assess the best size of rainwater harvesting tank as follows:

Basic Calculation:

The basic calculation just uses the supply of water from your roof to determine the best size tank for you. It does not factor in how much water you will use. In most cases it works fine. The calculation is as follows:

Footprint of property (m2) x Drainage coefficient x filter efficiency x annual rainfall x 0.05 = recommended tank size.

Whilst this seems complex, we can break it down easily:

Footprint of property: This is the length multiplied by the width of the building in metres.

Drainage coefficient: Not all of the water that hits the roof will get into the tank, some will evaporate, and some will splash over the gutters in a heavy downpour. The “drainage coefficient” takes account for this by suggesting 80% will get down the downpipes. Therefore, we use 0.8 for this figure.

Filter efficiency: Like the drainage coefficient, this is a number used to factor in that not all of the water will get through the filter, most filters have self-cleaning properties and some water will be lost during the self-cleaning. Our filters are 95% efficient. Therefore, we use 0.95 for this number.

Annual Rainfall: This is how much water in mm that you will get in a year, you can find this information on the met office website.

At this point the figure gives us the amount of water you will receive in the tank in one year. The 0.05 is to give us our final tank volume, which is 5% of a year’s water.

Here is an example:

Roof Area: 100m2

Drainage Coefficient: 0.8

Filter efficiency: 0.95

Annual Rainfall: 800mm

100 x 0.8 x 0.95 x 800 = 60,800 (litres or water received in a year)

60,800 x 0.05 = 3,040L

This shows that a 3000L tank would suit this particular building.

Intermediate calculation:

The Intermediate level of caculation takes the above basic method and adds the water consumption in to it. The basic calculation uses the supply of water off the roof but ignores your water usage. This isn’t something to worry about, the basic level is often very accurate but can recommend very big tanks in houses with large roof areas (especially for bungalows where usage might be low and roof area high) to take advantage of the intermediate calculation all you need do is estimate the water consumption as follows:

Number of people x water consumption per person x 365 x 0.05

For example: 4 people x 40L per person x 365 x 0.05 = 2920L

As you can see here, again, a 3000L tank would be fine and would work well with the previous example. However, if you only had 2 people your demand would be half, and therefore require 1500 litres of storage. It is always best to size the system with the potential occupancy in mind. So a good rule is:

2 bedrooms: 3 people

3 bedrooms: 4 people

4 bedrooms: 5 people

To discuss tank sizing further, feel free to contact us on: 01844 877030 and we can help you with your individual project alternatively our tank size calculator uses the basic method and will give you a good idea of the necessary tank size.

What’s next?

There are many things to consider when choosing a rainwater tank: