There are many calculators on the web to allow you to calculate your water usage:

BBC water calculator

The Consumer Council for Water , the "water watchdog" has a comprehensive spreadsheet calculator for all water companies. They also have a lot of information on water charges, droughts, the water network and many other aspects of the water supply.

Uswitch also have a calculator which can help you decide whether or not you would benefit from a water meter.

Most water companies also have their own calculators for you to use.