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Do you know where I can get a zipcode-to-county crosswalk online database? I have it for the state of Michigan but now I'd like one for the entire U.S. if possible. There are certain sites that have all states, but I would not want to download the files one state at a time.
There is a correspondence database:
It is also available for 2000 FIPS codes:
and for 1990 FIPS codes:
On occasion, a zip code will cross county boundaries. The results from this utility, will provide a weight that shows what proportion of the population of that zip code is in one county and what proportion is in the other(s). For the most part, zip codes map to single counties, but you'll be able to tell where it doesn't.
If you have trouble figuring out what to do, I'll walk you through one state. You can do all states at once. You just have to highlight them all.