I try to keep things simple and just say every gallon is approx. 4 litres. The 0.5 litres up or down realy don't make a difference. I mean, if they did, we'd all be calculating displacement by gravel, internal filters, heaters, fish and ornaments etc obsessively. A 1 gallon, regardless of whether it's US or UK, is realy too small for any fish IMO anyway.