Zip Code Population by County

Using Geocorr at the University of Missouri I got what I think you're looking for by using ZCTA as the "source" and county as the "target" example below, link is

mcdc.missouri.edu/.../geocorr2022.html

Dear Marti,

I'm not sure how Geocorr works.  Since I use R I download the mapping data and check for the tracts (or block groups in this case) that have an intersection with the chosen ZCTA geography.  This method uses only the "raw" mapping data so it is pretty complicated. Once I have the tracts (or block groups) that intersect the larger geography I add up the populations for the individual tracts (or blockgroups). Likely not too helpful  in this case.

Dave

Geocorr works by reducing both source and target geographies into collections of blocks -- the "atoms", if you will. We know the population of every block. So we can take all the blocks that are in ZCTA 21771 (say) and add up the populations of all those blocks that are also in county #24013, divide by the total ZCTA population to get the allocation factor, then repeat for any other counties that are partially covered by that ZCTA. That's why the allocation factors should always add up to 1, although there are sometimes rounding errors.

Yes entirely possible to do it yourself with block definition files and block populations. Did it in Florida after 2000 census to find the 1990 footprint of some  areas, for comparison, that had changed radically in 10 years. 
it makes more sense now than it will in 5 years as change continues

Yes entirely possible to do it yourself with block definition files and block populations. Did it in Florida after 2000 census to find the 1990 footprint of some  areas, for comparison, that had changed radically in 10 years. 
it makes more sense now than it will in 5 years as change continues