How often ZIP codes cross county lines

Why a ZIP code is not a boundary

The Postal Service assigns ZIP codes to delivery routes, the set of addresses one carrier serves. Routes follow roads and sorting facilities, not county or state lines, and the Postal Service has no reason to make them line up. A route that starts in one county and finishes in the next is a normal route.

To draw ZIP codes on a map at all, the Census Bureau builds ZIP Code Tabulation Areas: it takes each census block, looks at the ZIP codes of the addresses in it, and gives the block the most common one. The result is the closest thing to a ZIP code map that exists, and it is what nearly every ZIP map you have seen is drawn from. Because census blocks do respect county lines and ZCTAs are built from them, the Census Bureau can say exactly how much of each ZCTA lies in each county. That is the file these numbers come from.

Where it matters

For most uses, a ZIP that straddles two counties is fine. It matters when the county decides something: a sales tax rate, a contractor or trade licence, a franchise agreement written by county, a service area promised by county name, or a report that rolls ZIPs up to counties. In those cases the ZIP alone cannot tell you the answer for an address near the line, and a list that maps each ZIP to one county is quietly wrong for part of it.

The share varies a lot by state. The highest are South Dakota (57.1%), Puerto Rico (56.1%) and Mississippi (54.4%). South Dakota has the most ZIP code areas crossing a state line, 41. Hawaii and the District of Columbia have none split at all.

Every state

Each state page lists every split ZIP code area in it, with its counties and their shares of its land. A ZCTA on a state border is counted in both states, so the state rows add up to slightly more than the national total. The four smaller territories (American Samoa, Guam, the Northern Mariana Islands and the U.S. Virgin Islands) are in the national numbers and the audit tool but have no page of their own.

Method

1. Start from the Census Bureau's 2020 ZCTA to County relationship file, which has one row for each piece of a ZCTA that falls in a county, with the land area of that piece.
2. Skip the rows with no ZCTA, which describe the parts of counties no ZIP code area covers.
3. Count a ZCTA as overlapping more than one county when it has rows in two or more counties. Its share in each is that piece's land area over the ZCTA's total land area.
4. Count it as having at least 5% in a second county when the second largest of those shares is 5% or more, and as crossing a state line when its counties are in more than one state.

The script that does this is short and the input is public, so anybody can reproduce every number on these pages from the Census file.