Look at the box or packaging of the letter-size paper you're using to find the weight. Typical bond paper used in most home and office computers is 20 pounds per ream. Multiply the number of pounds by the number of ounces in a pound: 20 pounds times 16 ounces per pound equals 320 ounces. Divide the number of ounces by the number of letter-size sheets in a ream: 320 ounces divided by 2,000 pieces of paper gives a per-piece weight of 0.16 ounces. Count the number of pieces of letter-size paper in the document you need to mail. Multiply this number by 0.16 ounces to obtain the total weight.