Estimate weight of a letter based on number of pages?

To estimate the weight of a letter based on the number of pages, first look at the type of paper in use. The "weight" of a paper in the USA is expressed in pounds. The average piece of inkjet paper is 20lb paper, which is based on the weight of one ream (500 pages) of the base size (17"x 22") which makes 4 sheets of 8 1/2" x 11", letter sized paper. To convert to find the weight per piece of paper:

(pounds/sheets in a ream)*16 ounces= ounces per 1 sheet

Example:

For one sheet of standard 20lb 8 1/2 x 11 paper-

(20/2000)*16=0.16 ounces per sheet.

For USA domestic postage, a standard First Class stamp is for up to one ounce, or about 6 pages of 20lb 8 1/2" x 11" paper.