The weight of metric paper is given in grams per square meter (gms). By definition, one square meter is one A0 size sheet or 16 A4 size sheets. The weight of United States letter size paper is given in pounds per 500 sheet ream of uncut C size paper. For the letter size paper, a sheet is cut into 4 pages so that a cut ream of letter size paper, 500 letter size sheets, weighs 5 pounds if the paper is nominally 20 pound paper. Note that each type of paper has a different "base size," so 200 gsm of one paper will give a differnt Lbs/ream equivalent for a different paper. For instance: 20 lbs of Bond is almost the same as 28 of cover.