Just about every airline I've dispatched uses the standard weights for passengers of :
male - 88kgs
female - 70kgs
child - 35kgs
inf - 0 kgs
Some airlines though just use a standard "adult weight" of 80kgs.
Standard bag weights tend to differ though from airline to airline and whether that flight is international or domestic.
At the moment, my airline uses standard bag weights of 15kgs on all flights except for LHR-LOS-LHR where we use actual bag weights (for obvious reasons).
We have literally (last 4 days) just had a guy in weighing all the baggage containers as they come off the a/c to find out how inaccurate our bag weights are using the standard weights. Some flights have been having upto 4500kgs more weight than what the standard bag weights are.
Some of the a/c are able to weigh themselves, our A340's and some of the newer 744's do this, but a loadsheet is still required. The loadsheet is then cross checked against what the a/c thinks it weighs. There is a slight margin for discrepancies taken into account (not sure how much tho.)
To get our weights we work out the following (just a basic rundown)
Basic weight + catering + crew = Dry Operating Weight
Dry Operating weight + Pax + deadload (bags/freight/etc) = Zero Fuel Weight
Zero Fuel Weight + Fuel - Taxi Fuel = Take Off Weight
Take Off Weight - Trip Fuel = Landing Weight.
The a/c themselves get re-weighed every so often. The put a set of scales under each wheel and pull the a/c onto the scales. They take the readings and add them together. They then swap the scales around and re-weigh the a/c. This is done 3 times and they then take the average weight as the basic aircraft weight. (the people that have been weighing our bags also weigh the a/c using the same scales that they use to weigh the a/c).
B747skipper says that people on the ground take the best part of 1 hour to compute the info. Don't know which airline he works for, but I can produce a manual loadsheet for an A321/A320/B737 from scratch in less than 10 mins, and about 20mins for a B747 and for a computerised loadsheet, about 5 mins for shorthaul, 8-10 for longhaul. Sure we could do a rough calculation, but for legal reasons we can't dispatch an a/c on some rough calculations. The loadsheet is a legal document that has to be produced in accordance with the regulations of the particular airline and with the governing authorites (in the UK, the CAA) telling the f/deck crew how much the a/c weighs, what the trim setting is and how many pax are onboard along with some other bits of info.
It is technically illegal for an a/c to depart without a loadsheet being produced and a copy being held at the station of departure, and if anything were to happen to the a/c, the loadsheet and any other paperwork is confiscated and the dispatcher automatically gets suspended pending the investiagation. If it turns out the dispatcher made a mistake that caused the a/c to crash, they could end up in jail.
"She Rolls, 45 knots, 90, 135, nose comes up to 20 degrees, she's airborne - She flies, Concorde Flies"