paper weights

does anyone know where the paper weight value comes from… I have gone into a Kinkos and seen 20# paper, 60#paper, 80# paper, and 110# paper and some times the 60# is thicker than the 80#, it doesn’t seem to always make sense…what do the values really mean and how can some some be thicker than others even though their weights are less…any ideas?..thanks

It’s the “basis weight”

In US measurements for letter size paper its the weight of 500 sheets of uncut ‘C’ size (17"x22") - but each type of paper has different basis sizes too, so the basis weights are not universal across different paper types.

In metric, it’s the “gms” weight - or grams / sq meter, or one A0 sheet.

Here and here.