Metropolitan statistical areas are defined by the Office of Management and Budget as groups of whole counties around an urban core. Because ZIP codes follow delivery routes rather than county lines, mapping a ZIP to its metro area needs a crosswalk rather than a simple lookup.
MSA, micropolitan area and CBSA
A core based statistical area (CBSA) is either a metropolitan statistical area, built around an urban core of 50,000 or more people, or a micropolitan statistical area, built around a core of 10,000 to 50,000. Every CBSA is a set of counties, so a ZIP is in a CBSA when its county is. Counties outside any CBSA are simply “non-metro.”
Use the HUD ZIP-CBSA crosswalk
The HUD-USPS ZIP-CBSA crosswalk lists each ZIP code with every CBSA it touches and four ratios: residential, business, other and total address shares. It is updated quarterly. To assign one CBSA per ZIP, take the row with the highest ratio for the address type you care about, and record the file's quarter so the assignment can be reproduced.
Keep the edge cases
- ZIP codes that straddle two CBSAs should keep both rows if you are allocating counts.
- PO Box-only ZIP codes have no residential addresses; use the business or total ratio.
- CBSA definitions are revised periodically; note the definition year alongside the crosswalk quarter.
Quick check for one ZIP
Open the ZIP's page from the ZIP code lookup to see its county association, then find that county's CBSA in the current OMB delineation list. For anything more than a handful of codes, use the crosswalk.