Why ZIP codes have 5 numbers — and what they each mean

Esri zip code tool
ZIP code areas in New Jersey and New York Esri

Today marks the 52nd birthday of the introduction of ZIP codes in the US.

Although the explosion of business mail is mostly responsible for the Zoning Improvement Plan (ZIP), the idea started during World War II, when the first 2 numbers were introduced.

The initial zoning address system took effect in 1943 when thousands of postal employees left to serve the military during the Second World War, leaving the system understaffed and in need of simplification. To start, 124 of the largest US cities were classified with two numbers — the first identifying the city and the second the state. The numbers were intended to make sorting easier for less experienced employees.

The postal system implemented the three other numbers 20 years later in 1963, as mail volume grew.

What each number means

  • The first digit designates a broad area, which ranges from zero for the Northeast to nine for the far West.
  • The two following digits are the code of a central post office facility in that region.
  • The last two digits designate small post offices or postal zones.

To encourage people to use Zip codes, the Postal Service used a cartoon figure ‘Mr ZIP.’ Wikimedia

For example, in a 10014 ZIP code, the first “1” stands for Delaware, New York, Pennsylvania; the “00” stands for New York Sectional Center Facility and “14” for the part of Manhattan (Meatpacking and the West Village).

Six months after the implementation of ZIP codes in 1963, every address in the United States had been assigned one.

Additional numbers

USA Stamp 1973 ZIPCode 2
ZIP code stamp Wikimedia

In 1983, a hyphen and four digit code was added to some of the already existing ZIP codes. The ZIP+4 code was implementedwhen it became difficult to sort mail based only on traditional ZIP codes, but it never quite caught on. The development of sorting technologies shortly after its implementation made it mostly redundant.

The sixth and seventh numbers delineate a “delivery sector,” which might be an office building, a small geographical area, a few blocks or a large apartment building.

The last two numbers stand for a delivery segment which can be as restricted as the floor of an office building or a specific department in a firm.

Although the development of technologies in the last few decades has seriously diminished the amount of snail mail, ZIP codes are still used today for many purposes. A myriad of information — such as age, education level, gender ,or race — can be gathered about the population living in specific area based on ZIP codes.

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