or
lengthxwidthxdepthx6.25 gives you gallons. X that my 4.55 to give you litres.
do the measurements in feet so if you have say 30inch it's 2.5
or lengthxwidthxdepth divided by 1000 gives you litres. then divide by 4.55 gives you gallons.
do these measurements in centimetres.
and then if you want US gallons X the UK gallons by 1.20095