yf,

It's not really a dumb question because it's a pretty complicated process. Here's a short explanation:

The airlines use an average “summer weight” and “winter weight” for passengers which takes into account clothing and carry on luggage. They also have the option of using “child’s” weights for children. They know where everybody is sitting and/or they assume an average distribution when they calculate passenger weight cg. It’s negligible if a few people change seats or move around.

They weigh your luggage when you check in and they know the weight of any freight. They assign the luggage/freight to luggage “bins” and calculate it’s cg.

They add the passenger and freight weight/cg to the known Operational Empty Weight (OEW) and cg of the airplane to come up with a Zero Fuel Weight (ZFW) and cg. The only thing left to add is the fuel weight. They run calculations to be sure the overall cg stays within limits as the fuel is burned during flight.

They used to make us do these calculations manually but now it’s done mostly by computers.

It may seem a little imprecise, given all the variables, but the calculations are conservative and there is a pretty good margin for error.

Craig