For most US cities, I can quickly get a map zoomed right to the central city by going to most any mapping application and typing in a five-digit ZIP code that ends in “01.” That’s because the first three digits of any ZIP code are actually a meta-ZIP, best known by the Census Bureau’s term “ZCTA3.”
Back when ZIPs were assigned, the large post offices that served entire cities or broad rural areas were assigned these three-digit codes, and subareas within them were numbered off in roughly concentric order from the sorting center. ZIP code XYZ01, then, was usually at the then-principal post office, either right downtown or by the railyards where mail was usually offloaded then.
The Post Office assigned these codes in an ascending order following a meandering east-west path across the Lower 48, beginning in New England and ending in the Northwest. Thus, it’s not terribly difficult to remember a city’s ZCTA3 — since they’re systematic, it’s easier than memorizing cities’ telephone area codes (a taxonomy that, while having its own charming history, is increasingly irrelevant).
- Boston 021
- New York City 100
- Pittsburgh 152
- Philadelphia 191
- Washington 200
- Raleigh 276
- Durham 277
- Atlanta 303
- Miami 331
- Minneapolis 554
- Chicago 606
- Denver 802
- Los Angeles 900
- Portland 972
- Seattle 981
BUT there’s one (one!) exception that thwarts my little mnemonic: San Francisco. Its ZCTA3 is 941, but there is no 94101; the lowest-numbered ZIP in town, centered on the main post office by the Civic Center, is 94102. Who stole 94101?!
Postscript via Eric Fischer, proud owner of a pre-ZIP postal map:
@paytonchung And I guess the real answer is the note that zone 1 is "Main Office Boxes." The old main post office moved, so 94101 is no more
— Eric Fischer (@enf) March 15, 2016
west north 