County ZIP Codes

Problems and Solutions with the USPS County ZIP Codes

ZIP codes are mail routes, not geographic boundaries. This creates some problems in the system when it is used for purposes other than delivering mail. For example, one ZIP code can cross into multiple cities and counties. The county ZIP code in one county could be the same as the county ZIP code in another county.

Because of the deficiencies in the system, most ZIP code database vendors, including the USPS, will define a primary county for a ZIP code. In basic databases, this is the only county that will be included - even if the ZIP code crosses into multiple counties.

Many companies need a ZIP code database that includes all the county ZIP codes. ZIP Code Download has a product called the Multi-county ZIP code database for this purpose. This database includes all the counties included in a ZIP code.

ZIP code databases similar to ZIP Code Download's database usually don't include cities in the database. The simple reason as to why they are not included is because it is impossible to link a city with a county based on ZIP code data. The city, like the county, can cross into multiple ZIP codes, which creates a mess. It is better to keep this information separate, so most companies will include a 5-digit ZIP code database with the county data so that customers will also have the city.