The postage rate for a first-class letter sent from the United States to the United Kingdom is $1.15. This rate applies to letters weighing not more than 1 ounce and the content value not exceeding $400.
To determine the international pricing for first-class letters that weigh over 1 ounce, the customer can refer to the USPS website, which provides a postage price calculator. Letter-size mail that exceeds 3.5 ounces in weight is charged first-class mail international flat-size rates. The dimensions of letter-size mail should not exceed the following: 11 1/2 inches in length, 6 1/18 inches in height or 1/4 inch in thickness.