[scrumper]

You measure up front and plan. Doing a weight and balance calculation is a standard (and critically important) part of flight planning. For light aircraft in particular it is _really_ important because there is very little margin for error. With light private aircraft you're weighing yourself, your passengers, your bags, and the fuel load and you're only taking off if you're within a safe envelope.

I'm not familiar with airline operations because I've never flown anything bigger than a Cessna 172, but from what I gather the airlines assume an average per-passenger weight, and account for number of bags and the ground crew will distribute in the holds accordingly. Ever seen flight crew moving passengers on little regional jets? It's to make the weight distribution safer. The smaller the plane, the bigger the effect of a few passengers.