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These last 4 digits represent specific delivery routes within delivery areas. This extra detail means an even more precise matching to a more granular level. It could represent a handful of houses on one side of a street, or even a single building that receives a high volume of mail.

Census tracts have well known/exact boundaries. ZIP codes are groups of lines whose exact structural definition is not officially established.

The Census Bureau offers a few statistical products by ZIP Code through our economic surveys. These data are available through data.census.gov. Please contact the US Postal Service for additional information about ZIP codes.

Looking up a ZIP+4 with the USPS ZIP Code? Lookup Tool Enter the address you want to look up in the ?Street Address? field. You can also enter a city and state or a ZIP code to narrow down your search. Click the ?Find? button to search for the ZIP+4 code associated with the address.

Because ZIP+4 codes are based on delivery routes, they change when the postal delivery routes are updated by the USPS. That's why ZIP+4 codes can change as often as once a month, since there are several factors that can result in the USPS updating its delivery routes.

The check digit is calculated by adding up the digits of the zip code and seeing what would have to be added to the sum to get a multiple of 10. For example, the check digit for the zip code 51220 would be 0 (as 5 + 1 + 2 + 2 + 0 = 10), while the check digit for 82125 would be 2 (as 8 + 2 + 1 + 2 + 5 = 18).

Writing the wrong ZIP Code with the right address usually causes the letter to be delivered about a day later than if the ZIPCode were correct. As long as the mailing address is correct, the letter will be delivered.

Your ZIP+4 is a basic five-digit code with four digits added as an extra identifier. It helps to identify a geographic segment within the five-digit delivery area, such as a city block, a group of apartments, an individual high-volume receiver of mail, or a post office box.