I couldn't find one that covered everything I needed, so I used CalcLite, which is free, and wrote my own to calculate the cg, weight etc for the Arrow I fly. It has enough to indicate automatically whether it is within the cg envelope, including the cut-off corner. It can cope with imp gals (written on the tank for tabs level), us gals (max fuel tank capacity given in this, plus fuel burn figures), and litres, which are the units most commonly used when filling up.
It also covers duration / duration and fuel requirements for the trip.
There's a bit of work needed to set it up, but it does work well, and it's both adaptable and free of charge.