Miscalculating the number of stamps you need to send an article of mail can result in returned, delayed mail and a major hassle; using too much postage, however, is a waste of money. Fortunately, estimating the proper postage for your letter-size paper doesn't require running out to buy an expensive postal service scale—it just requires a little information about what you're sending and a brief calculation.
Determine the weight of your paper. This "weight," generally listed on the package, is not the weight of a single piece of paper. Rather, it is the weight of a ream—500 sheets— of paper, which is then cut into individual pieces of paper. On average, ink jet paper is 20 lbs. and fits four pieces of paper to a sheet.
Divide the "weight" by the number of pieces of paper in a ream. For example, an average sheet of 20 lb. ink jet paper fits four pieces of paper to a sheet, and there are 500 sheets in a ream. In this case, you would divide 20 by 2,000, as there are 2,000 pieces of ink jet paper in a ream.
Multiply this number by 16, the number of ounces in a pound—this final number is the weight, in ounces, of a single letter-sized piece of paper. For example, the 20 lb. ink jet paper comes out to 0.16 oz. per piece.
Calculate how many stamps you'll need. A standard stamp for U.S. domestic mailing is worth about 1 oz., meaning one stamp would cover about six sheets of paper from the example.