Each individual aircraft is weighted at specified intervals (several years). The weighting includes determining the Center of Gravity and it’s results are included in the documents of an aircraft. For each flight actual(*) mass and position of crew, fuel, pax etc. are added to this basic figure to determine the actual mass and CofG (expressed in %MAC) of the aircraft.
* In some cases an aproved standard mass can be used instead of the actual mass. That’s why every pax isn’t weigted before flight.