In hindsight, my comment was made too simply. I should have included, volume required.
Exactly this-
Originally Posted by mechta
The volume of fuel that can be put into it will stay the same if we assume no tank expansion, but the mass will depend on fuel density.
We need fuel by mass, despite delivery by volume. I flew with a fella who had difficulty accepting density values. When I tried to explain to him, it went straight over his head. His 'assumption' generally being that there must be more fuel on board, despite what the gauges say, because the book says the tanks hold x lbs.
I hope that makes sense.