5-Digit ZIP Code Tabulation Area, Illinois

ZIP Code Tabulation Areas (ZCTAs) are approximate area representations of U.S. Postal Service (USPS) fivedigit ZIP Code service areas that the Census Bureau creates using whole blocks to present statistical data from censuses and surveys. The Census Bureau defines ZIP Code Tabulation Areas by allocating each block that contains addresses to a single Code Tabulation Areas, usually to the ZCTA that reflects the most frequently occurring ZIP Code for the addresses within that tabulation block. Blocks that do not contain addresses but are completely surrounded by a single Code Tabulation Areas (enclaves) are assigned to the surrounding ZCTA; those surrounded by multiple ZCTAs will be added to a single ZCTA based on limited buffering performed between multiple ZCTAs. The Census Bureau identifies five-digit Code Tabulation Areas using a five-character numeric code that represents the most frequently occurring USPS ZIP Code within that ZCTA, and this code may contain leading zeros.

There are significant changes to the 2010 Code Tabulation Areas delineation from that used in 2000. For 2010 only legitimate five-digit areas are defined so there is no longer full nation-wide coverage. The 2010 ZCTAs will better represent the actual Zip Code service areas because the Census Bureau initiated a process before creation of 2010 blocks to add block boundaries that split polygons with large numbers of addresses using different ZIP Codes.

Data users should not use Code Tabulation Areas to identify the official USPS ZIP Code for mail delivery. The USPS makes periodic changes to ZIP Codes to support more efficient mail delivery. The Code Tabulation Areas process used primarily residential addresses and was biased towards ZIP Codes used for city-style mail delivery, thus there may be ZIP Codes that are primarily nonresidential or boxes only that may not have a corresponding ZCTA.

ZIP Code Tabulation Area Codes - The Census Bureau identifies 5-digit ZCTAs using a five-character numeric code. For ZCTA codes that reflect the 5-digit ZIP Code, the last two characters of the ZCTA code will be numeric. For example, the ZCTA code "00601" represents the 5-digit ZIP Code 00601. The ZCTA delineation process did not recognize ZIP codes ending in "00", such as "29000", as valid 5-digit ZCTA codes. ;