Here is the question I was looking for (it didn't answer my question though), but this is specific to Boeing 787.
Is there any regulation to carry certain amount (may be %) of fuel aircraft should carry when its flight plan is prepared?
Note: We had argument that aircraft should carry 150% of fuel of it's planned travel, then someone bought argument of hijacking planes and said that 15 to 30 minutes of extra travel time fuel they have to carry.