Litres are international as they are a metric measurement so a litre in the US is the same as the UK, France etc.
Gallons and pints are imperial measures so these are the ones that can vary. A UK gallon is bigger than an American gallon (surprisingly).
If you want conversion table I've got one that does loads of stuff, more than any sane person would ever need, but I dont know how to post it up on the net or what rules might restrict this. Email or PM me if you'd like a copy- I know how to do that!!
Oh BTW if you are wanting the conversion so you can calculate how many fish you can have, don't forget to take off between 5-7% for the substrate and decor