Take it that Bertie wanted to have this information as an aide to flightplanning and more in particular the weight and balance thing.
Bertie, if you have a look in the W&B part of your POH you are likely to find a table with volumes, weights and moments which will help (as others have mentioned keep an eye out for the US/IMP gallon trap!)
And if you forget what the weight was (as I always do as it comes under the heading of rarely immediately needed info) have a look in the info section of any good VFR guide. A fond of knowledge.
Have fun!
FD