Second part of ZIP code

Mar 3, 2026

Updated: March 3, 2026

The second part of a ZIP code refers to the additional four digits that follow the primary five-digit ZIP code in the United States. This format is commonly known as ZIP+4, which is the main variant of this term.

The first five digits of a ZIP code identify a general delivery area, including the region, the Sectional Center Facility (SCF), and the associated post office or delivery zone. The ZIP+4 extension further refines the location: the sixth and seventh digits narrow it to a smaller area such as a city block or large building, while the eighth and ninth digits identify a specific delivery segment, such as one side of a street, a floor, or a group of apartments.

ZIP+4 codes were introduced to improve mail sorting accuracy and delivery speed. They help postal services distinguish between addresses that share the same five-digit ZIP code but differ in final destination.

Including the second part of a ZIP code is not always mandatory, but it significantly improves address precision. Many address validation systems attempt to append or confirm the ZIP+4 when USPS reference data is available, and sufficient address information is available.

In data management and logistics workflows, ZIP+4 enhances matching accuracy, reduces ambiguity, and supports more precise geocoding. Accurate ZIP+4 assignment depends on high-quality address reference data and validated street-level information.

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