The value of the postage, or the number of stamps, required to send a letter varies according to the weight, shape and destination of the letter. The website of the United States Postal Service provides a calculator for determining the cost of sending a letter.
The Postage Price Calculator is accessed by clicking on the “Calculate a Price” tab on the USPS home page. The calculator asks whether the destination, which may be domestic or international. It also asks for the shape of the letter. Once this is entered, the calculator asks the weight, in pounds and ounces, of the letter. To weigh a letter, use a postal scale or a kitchen scale.
After the weight is entered, the calculator asks the user to select one of four properties that best characterizes the letter. These are "letter," "square envelope," "envelope containing a rigid object" and "odd shaped envelope." The lowest rate for a particular weight applies to the standard "letter." Postage for letters with one of the other properties costs more.
The final prompt asks whether the postage is to be paid by affixing a stamp or using a postal meter. The cost of sending a metered letter is slightly less than the cost of sending a stamped letter. As of December 2014, the cost of sending a one-ounce stamped letter within the United States, including U.S. territories and possessions, was $0.49.