This is a very general question, but if the standard paper size is around 80 GSM's in the UK:

A4 - Printing Paper - 80 GSM.

US standard sizes of paper are different so these will vary.

Depends on the type and thickness of the paper, which will depend on the paper weight.

There are other variables which define the weight of the paper such as how the fibres are held together and also the substance which holds the material together.

Recent paper has also got substances such as polypropylene and polyethylene inside the material. Having these substances will also effect the paper weight.

The best weight to weigh the paper is to get an electronic weighing machine which will provide the weight in 2 decimal points to the weight of the paper.