PoolStore's handy volume calculator will help you.
Type in the length, width and average depth of your pool into the boxes below and click on the 'calculate' button and the volume of your pool in cubic metres and gallons will be displayed.
Calculating the average depth of your pool is not easy because most pools have a shallow end and a deep end with a slope in between. If area of the shallow end about the same as the deep end then take the depth half way along the slope as the average depth. You can input the figures in feet or metres.