A territory map built from ZIP codes carries a quiet assumption: that a ZIP code sits inside one county, one tax jurisdiction, one licensing area, one franchisee’s patch. We wanted to know how often that is true, so we counted.
The answer, from the Census Bureau’s own 2020 relationship file: 10,186 of 33,791 ZIP Code Tabulation Areas, or 30.1%, overlap more than one county. 137 of them cross a state line.
What we measured
The Census Bureau publishes ZIP Code Tabulation Areas (ZCTAs), its polygons approximating the areas ZIP codes serve, along with a relationship file that records how much land each ZCTA shares with each county. We read that file and nothing else. It is public, and anybody can repeat this in an afternoon.
Two things to keep in mind. A ZCTA is not the same as a postal ZIP code: ZIP codes are delivery routes, and some of them, such as post office box banks, have no ZCTA at all. And our shares are land area, not people, so a sliver of farmland and a sliver of downtown count the same.
Most splits are not slivers
Edge noise would be a fair objection: a boundary drawn from census blocks will clip a neighbouring county by a few acres. So we raised the bar.
| Land in a second county | ZIP areas | Share |
|---|---|---|
| Any | 10,186 | 30.1% |
| At least 5% | 7,674 | 22.7% |
| At least 10% | 6,266 | 18.5% |
| At least 25% | 3,359 | 9.9% |
| At least 40% | 1,018 | 3.0% |
One ZIP area in ten has a quarter or more of its land in a second county. That is not a rounding problem. 2,546 ZIP areas touch three or more counties, and four of them touch six.
Where you are matters
The national figure hides a split down the middle of the country.
In Mississippi, South Dakota and Georgia, more than half of ZIP areas overlap more than one county. Georgia has 159 counties, many of them small, and rural delivery routes were never drawn with them in mind.
In Massachusetts, Maine, Connecticut and Rhode Island, fewer than one in twenty does. New England ZIP codes grew up around towns, and towns are what the county lines were drawn around too.
So a territory scheme that works without complaint in Hartford can fall apart the week it is copied to Atlanta. Every state’s numbers, with the list of split ZIP areas, are on its own page.
Why a county line matters to a territory
A county line is often where something else changes: a permit office, a tax rate, a licensing board, a franchise agreement that names counties, a crew that does not cross the river. When the territory is a list of ZIP codes and the rule that matters is a county, every split ZIP is an address you will answer for on the wrong side of the line some of the time, and the spreadsheet cannot tell you which addresses.
This is the case for deciding on the address rather than on the code, which the earlier piece on ZIP codes goes through in detail: geocode the address to a point and test the point against the boundary you actually govern.
Check your own list
If your territories live in a spreadsheet of ZIP codes, the ZIP list audit runs these checks against it in your browser: ZIPs that overlap more than one county, ZIPs on two territories, ZIPs with no Census area, and codes that lost their leading zero in Excel. The file never leaves your computer and there is nothing to sign up for.