Only one stamp is needed to mail a normal letter, assuming the stamp is either a forever stamp or worth 49 cents in the continental United States. The cost of postage to mail a letter was increased by three cents in January 2014.
A typical letter weighs 1 ounce or less. If the letter weighs more than 1 ounce, additional stamps are needed; each additional ounce costs 21 cents more. As of 2014, it is necessary to use one stamp worth 49 cents to mail a normal letter or the old 46 cent stamps plus three 1 cent stamps. Forever stamps indicate the current cost of mailing a typical letter regardless of when they were purchased.