Illinois

The number of ZIP codes per state varies widely. Texas and California have the most with over 2,500 each, while smaller states like Delaware and Rhode Island have fewer than 100. The count depends on population density, land area, and how the USPS has organized delivery routes.

Each state is assigned specific ZIP code ranges. For example, New York uses 100xx-149xx, California uses 900xx-961xx, and Texas uses 750xx-799xx. These ranges generally follow the east-to-west numbering pattern established when ZIP codes were introduced in 1963.

States share ZIP code prefixes because the first three digits identify a Sectional Center Facility (SCF), which may serve areas in multiple states. Border regions often share mail processing facilities, leading to overlapping prefix assignments.

The US Census Bureau uses ZIP Code Tabulation Areas (ZCTAs) to associate demographic data with postal areas. ZCTAs approximate ZIP code boundaries using census blocks and provide population, income, housing, and other statistics at the postal code level.

ZIP codes are delivery routes defined by the USPS for mail delivery, while ZCTAs (ZIP Code Tabulation Areas) are geographic areas defined by the Census Bureau for statistical purposes. ZCTAs approximate ZIP code boundaries but may not match exactly, especially in rural or less populated areas.