ZIP (Zone Improvement Plan) Code is a system of 5-digit codes, established in 1963, that identifies the individual post office or metropolitan area delivery station associated with an address. The first three digits identify the delivery area of a sectional center facility or a major-city post office serving the delivery address area. The next two digits identify the delivery area of an associate post office, post office branch, or post office station.

ZIP+4 is a nine-digit numeric code, established in 1981, that is composed of: (a) the initial five-digit code (see above) and (b) the four-digit expanded code: the first two additional digits designate the sector (a geographic area) and the last two digits designate the segment (a building, floor, etc).

Source of definition: U.S. Postal Service ZIP Code FAQs

Date of definition: July 2013

Note on two types of ZIP codes: There are two types of ZIP Code reported on IECAM: postal ZIP code and census ZIP code or ZCTA.

  • Postal ZIP code. This ZIP Code, used by the U.S. Postal Service, is what people generally think of when they think of ZIP codes. However, the postal ZIP code is a delivery route. It is NOT a geographic area (although it sometimes approximates a geographic area). Its purpose is to foster more efficient mail delivery. There are many parts of the US that do not have ZIP codes because they are not on postal delivery routes. Note that ZIP codes change every year as new ZIP codes are created and old ZIP codes are eliminated, combined, or split.
  • Census ZIP Code, or ZIP Code Tabulation Area (ZCTA). This ZIP code is used by the U.S. Census Bureau. It is an artificial area created by the Census Bureau using Census Blocks. The ZCTA is very roughly similar to the geographic area of a corresponding postal ZIP code (for only those postal ZIP codes that approximate geographic areas)

Note on yearly changes in ZIP Codes Postal: Postal Zip codes change every year as new ZIP codes are created and old ZIP codes are eliminated, combined, or split.

Note on presentation of early care and education data by Zip Codes Postal: IECAM presents data by ZIP Codes Postal for all years for the number and capacity of early care and education sites (PFA, Head Start, child care). These data are user-reported. IECAM makes an effort to correct obvious errors in user-reported postal ZIP codes. However, IECAM cannot guarantee that all postal ZIP codes are accurate.

Note on presentation of demographic data by Zip Code Postal: Because the Census Bureau does not estimate data by postal zip codes (because they are not geographic areas), IECAM cannot present demographic data by Zip Codes Postal.

Note on comparing data between “ZIP Codes Postal” and “ZIP Codes ZCTA 2010”: Given that the postal ZIP code is a delivery route and the ZCTA is a geographic area that artificially tries to approximate the postal ZIP code, these two ZIP code types are not precisely comparable. For example, a child care center with a postal ZIP code of 618xx may not be located in the ZCTA 618xx.

Given that Postal ZIP Codes change every year and that ZCTAs do not, there might be Postal ZIP Codes that have no corresponding ZCTA, and vice versa.

For these reasons, IECAM does not recommend comparing early care and education service data (i.e., number of sites or capacity of PFA, HS, child care) by ZIP Codes Postal with ZIP Codes ZCTA.

Note on comparing data  by “ZIP Codes Postal” across years: Comparing early care and education service data (i.e., number of sites or capacity of PFA, Head Start, child care) for postal ZIP codes across years is possible. However, note that there are two serious concerns with such comparison. First, because postal ZIP codes change every year, no two years’ ZIP code lists are the same. Second, because the postal ZIP codes are user-reported and because it’s not possible to correct all user-reported errors, IECAM does not guarantee that all user-reported postal ZIP codes are correct.

Web Resources: Geographic Regions