028 9261 1077

Selecting your Tank


The size of a rainwater holding tank must match the demand for water with its availability as closely  as possible.  BS 8515:2009  provides an authoritative and industry-approved means of calculating the optimum size of tank for household water use.

Unlike other industries it is better to reduce the size of the tank rather than over size it if in doubt as to the requirement.  This is due to the benefit of allowing the tank to overflow at least twice a year to flush out floating debris.  The tank size chosen must therefore be a balance between rainwater supply and water demand.

In order to correctly size the tank, the capacity should be the lesser of either the rainwater supply or the water demand.  A worked example demonstrating this principle is shown below.

STEP 1: Working out your annual rainwater supply


Formula with example:

Tank size (litres) = Roof area m2 X Drainage Coefficient X Filter Efficiency X Annual rainfall mm X 5%
5,265 = 130 X 0.8 X 0.9 X 1125 X 5%

Roof Area
This is the width and length of your roof in m2.  Remember you do not have to collect water from the entire roof.  This can help to reduce installation costs.


Drainage Coefficient 

It is impossible to collect every drop of rain that falls on your roof.  Light rainfall will only wet your roof and then evaporate and heavy rainfall can overflow gutters.  A drainage coefficeient is used to adjust the tank size calculation to allow for this.

Roof Type Drainage coefficient
Pitched roof tiles 0.75-0.9
Flat roof smooth tiles 0.5
Flat roof with gravel layer 0.4-0.5


Filter Efficiency 

If debris reaches the filter, it momentarily reduces capture of water and is used to remove the debris to the overflow.  During heavy rain, high flows of rainfall will also bypass the filter to overflow.  As a result, 90% of rainfall is caught meaning the filter coefficient is 0.9.

Annual Rainfall 

This can vary dramatically over a relatively small area so a reading (in mm) within 10 miles of the property is preferable.  The local Environment Agency or Met Office can supply rainfall data or you can use the regional guide figures provided in the table below.  In the ‘annual rainwater supply example’ above we have used the average figures for the UK.

Region Average Rainfall (mm)
UK 1125
S England 781
N England 944
Wales 1433
Scotland 1520
NI 1111
ROI E 875 (Irish Met service)
ROI W 1125



Based on BS8515 the rainwater tank must be big enough to hold 5% of the annual rainwater yield, or 5% of the annual non-potable water demand, whichever is the lesser.

STEP 2: Working out your annual rainwater demand?


Formula with example:

Tank size (litres) = Average Usage  X People in Household X days/year X % harvestable X 5%
5,015 = 150 X 4 X 365 X 45.8% X 5%

Average usage
On average, each person in the UK uses around 150 litres of water every day and 45.8% of this total could be rainwater instead of mains water.

What is the right tank for you?
The capacity selected above should be the lesser of the two values. ie, 5,015L (as this is less than 5,265L).

Again following the same rule,  the specific tank should be selected based on your annual rainwater supply and your annual rainwater demand.