ZIP Code 101
Everything you need to know about ZIP codes — their structure, types, history, and how they power mail delivery across the United States.
What Is a ZIP Code?
ZIP codes are 5-digit codes used by the USPS to route mail efficiently. Learn what each digit means and how the system works.
ZIP+4: The Extended ZIP Code Explained
ZIP+4 adds four digits to the basic ZIP code, pinpointing a delivery segment as small as one side of a city block.
A Brief History of ZIP Codes
From postal zones in the 1940s to today's Intelligent Mail Barcode, trace how the ZIP code system evolved over 60 years.
How USPS Sorts Mail by ZIP Code
Follow a letter from mailbox to doorstep and see how optical scanners, barcodes, and sorting machines use ZIP codes at every step.
ZIP Code Types: Standard, PO Box, Unique, and Military
Not all ZIP codes are created equal. Learn the four types and why the distinction matters for geocoding and analytics.
Reading a ZIP Code: What Each Digit Tells You
The first digit reveals the region, the next two identify the sorting facility, and the last two pinpoint the delivery zone.
ZIP Codes vs. ZCTAs: Understanding the Difference
ZIP codes are delivery routes, not geographic areas. The Census Bureau's ZCTAs are the closest approximation — here's why the distinction matters.
How ZIP Codes Are Assigned and Changed
The USPS adds, splits, and retires ZIP codes as communities grow and shrink. Here's how the assignment process works.