wiggy, you can put your tin hat back on the shelf.
You're right.
It's the weight of the fuel that determines whether you can carry enough 'energy' to take you from A to B plus diversion, not the volume.
So the weight is what you want to know, and what is displayed (derived from fuel tank level sensors + fuel density sensors).
Still, it's the volume that determines whether it actually fits in the tanks.....
Wasn't there a thread recently about whether you were billed for litres or kilograms?