Ask someone to draw their ZIP code on a map and they will sketch a neighborhood. The Postal Service never did. A ZIP code is a set of delivery routes and post office boxes, and it has no official boundary at all. That surprises people, and it is the source of most confusion about crime data "by ZIP".
What a ZIP code really is
The US Postal Service created ZIP codes in 1963 to sort mail. Each one is a collection of carrier routes assigned to a post office. Routes can be added, split, or moved without anyone outside the post office noticing, and some ZIPs are a single building or a single organization with no residents at all. There is no ZIP boundary file from USPS because, from the Postal Service's point of view, there is nothing to draw.
What a ZCTA is
The Census Bureau needed a way to report statistics for the areas people think of as their ZIP code. So it created ZIP Code Tabulation Areas, or ZCTAs. A ZCTA is a real polygon built from Census blocks: each block is assigned to the ZIP code used by most of the addresses inside it, and the blocks sharing a ZIP are stitched into one area. ZCTAs are published after each decennial census, so the current set is the 2020 one, and there are about 33,800 of them.
For almost every populated place, the ZCTA carries the same five-digit code as the ZIP and covers roughly the same ground. That is why data providers, including CrimeScore, score ZCTAs and call them ZIP codes. It is the only way to draw a boundary and count what happens inside it.
Where the two disagree
- ZIPs with no ZCTA. Post office boxes, single-building ZIPs, and military ZIPs have no residential blocks to build from, so they have no ZCTA and no score. There are roughly 41,700 ZIP codes in use and about 33,800 ZCTAs, and the gap is mostly these.
- Edges. A block on the border between two ZIPs goes to whichever ZIP most of its addresses use. Your street may be in ZIP A for mail and ZCTA B for statistics.
- Timing. USPS changes ZIPs continuously; ZCTAs update once a decade. A ZIP created in 2023 has no ZCTA until the next census.
- Cities. A ZCTA does not respect city limits. CrimeScore assigns each ZIP to the city or place holding most of its land area, which is why a suburban ZIP can appear under a neighboring town's name.
Why it matters for crime data
Crime incidents are points. To count them "by ZIP" you need a polygon, and the polygon you choose changes the count. Two sources using different boundary files will report different numbers for the same code, and neither is wrong. What matters is that a source tells you which boundary it uses and applies it consistently over time so the monthly series is comparable.
CrimeScore uses 2020 ZCTAs for every ZIP and counts the same way every month. County scores use county FIPS boundaries, which are stable and well defined. When you look up a ZIP on this site, the page names the city and county the Census assigns it, and the methodology states the boundary rule.
Practical advice
If you are building a product that shows crime by ZIP, store both the user's postal ZIP and the ZCTA you scored, and be honest in the interface that the score describes an area, not an address. If you need something more precise than a ZIP, the answer is not a smaller ZIP; it is a different geography.
Browse scores by state and city, or look up any ZIP from the home page.