If you don't have this piece of information
then you need to calculate it. This is easy enough if the pool
is a nice rectangular or circular shape. If however you have
a pool that is a complicated shape then it is much more difficult.
(Serves you right if you had a guitar shaped pool installed when
that record went platinum!)
Here is a resource to help you and to
convert units into those most useful to you. As mains water is
supplied in Britain in cubic metres, start by measuring in metres
to obtain an answer in cubic metres. Then use the calculator
to convert this figure into litres, imperial gallons or US gallons.
Rectangles and Squares
If your pool has a sloping bottom, then take the deepest measurement
(D2) and the shallowest (D1). If there are different levels,
then you need to do several calculations of area for each depth
and add them up at the end.