One First-Class postage stamp can mail three sheets of 20-pound bond paper inside of a number 10-size envelope, which is 4 1/8 inches by 9 1/2 inches. This piece of mail weighs almost one ounce, assuming the paper is no larger than 8 1/2 inches by 11 inches.
Heavier sheets of paper, such as 24-pound bond, do require the extra postage for a letter that weighs between 1 and 2 ounces. A number 10 envelope that weighs heavier would also add a fraction of an ounce to the weight. If the letter contains more than three sheets of paper, it is recommended to affix extra postage for a 1- to 2-ounce letter or to have the letter weighed at the post office for correct postage.