The number of zip codes in the United States periodically changes as the U.S. Post Office finds it necessary to add new codes in response to increasing mail volume. According to the U.S. Census Bureau and other sources, there are at least 40,000 five-digit zip code areas in the U.S. In a period of one year, November 2006 to 2007, around 200 zip codes were added in the U.S.
The zip code, an acronym for Zoning Improvement Plan, became necessary in the early 1960s as business mail increased to such a point that it was nearly 80 percent of mail traffic. An Advisory Board of the USPS Department recommended that a coding system be developed to facilitate the speed of mail handling by increasing the use of machinery to sort the mail. The particular system, the Zip Code system, began on July 1, 1963.
Postal zones, already in place since the early 1940s, assisted in developing the new zip codes. The new system required that the entire mail system be realigned. The Metro System involved the installation of mail transportation centers in 85 cities in the United States. Next over 550 sectional centers were developed. Finally the particular codes were assigned to each center and then the particular addresses each served.
By the start date of 1 July 1963, each address in the U.S. had an assigned zip code. The first digit referred to one of ten geographical areas of the U.S.; the next two digits indicated a population center. The last two digits divided the population centers into small postal centers or zones within larger cities. Initially, no one was required to use zip codes.
In 1967, the U.S.P.O. required those who mailed items by second and third class mail to presort by the five digit code. In 1978, the postal system deemed it necessary to add a hyphen and four more digits to zip codes to speed the processing of the ever-increasing volume of mail. The resulting zip+4 system was in place by 1983. The additional four digits allowed for the mail to be sorted according to a city block or a particular office building. Like its predecessor the five-digit zip code, the zip+4 reduced the number of times a piece of mail has to be handled.