How U.S. ZIP codes work, digit by digit

ZIP codes. 7-minute read. Updated 2026-10-10.

ZIP stands for Zone Improvement Plan. The five-digit system went live nationally in 1963, replacing a patchwork of two-digit city zones. Six decades later it underpins sales tax, insurance pricing, delivery promises and a good share of the world's marketing data.

Reading the five digits

1. First digit: the national area. The country is split into ten groups numbered 0 to 9, running roughly from the Northeast (0) to the West Coast, Alaska and Hawaii (9). Alabama falls in group 3, along with much of the Southeast.
2. First three digits: the sectional centre. Together the first three digits identify a sectional centre facility, a large processing plant that serves a region. Codes beginning 361 are processed for the Montgomery area.
3. Last two digits: the delivery area. These point to a specific post office or delivery zone inside that region.

So 36118 reads as: Southeast group (3), Montgomery sectional centre (361), delivery code 18.

ZIP+4

Introduced in 1983, the four extra digits after the hyphen narrow delivery to a segment such as one side of a city block, a floor of an office building, or a large single recipient. Bulk mailers that pre-sort to ZIP+4 earn postage discounts because the machines have less work to do.

Why ZIP codes are not boundaries

A ZIP code is a set of delivery routes, not a polygon. Routes change when new subdivisions open, and a single code can cross city or even county lines. The Census Bureau publishes ZIP Code Tabulation Areas (ZCTAs) to approximate ZIP areas for statistics, but they are not identical to the postal codes. Anyone pricing insurance, setting delivery fees or drawing sales territories should know which of the two they are using.

How businesses put ZIP codes to work

• Sales tax: local rates often vary within a ZIP, so tax engines use the full address, with ZIP as a first filter.
• Delivery promises: retailers estimate shipping times from origin and destination ZIP pairs.
• Territories: franchise agreements and sales teams split markets by ZIP lists because they are easy to administer.
• Audience targeting: ad platforms allow ZIP-level location targeting, and direct-mail campaigns are priced per ZIP route.

Try it

Our ZIP and postal code explorer looks up place names and coordinates for U.S. codes and dozens of other countries, and measures the straight-line distance between any two codes.

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