One first-class postage stamp is sufficient to mail three sheets of 20-pound bond paper plus a number 10 business-size envelope. This letter weighs almost 1 ounce, assuming the paper is no larger than 8.5 inches by 11 inches.
Three sheets of heavier paper, such as 24-pound bond, require the extra postage needed for a letter weighing between 1 and 2 ounces. A heavier-than-usual number-10 envelope also adds a fraction of an ounce to the weight. If a letter contains more than three sheets of paper, it is best to affix the extra postage required for a 1- to 2-ounce letter or have the letter weighed at the post office.