There's a little more to it than you might think.
County data cannot be determined by just the ZIP Code.
Contrary to popular belief, ZIP Codes are not shapes. They are lines or routes traveled by mail carriers. They are allocated, changed, and even retired according to the needs dictated by efficient mail delivery. This means that ZIP Codes do not line up with municipal boundaries like those of a city, county, or even a state. There are even some ZIPs that cross as many as six counties and some that cross state lines!
There are many reasons for wanting to determine the county of a particular address, including:
The only way to properly determine which county a particular address is found in is to perform address standardization and verification against the official file, such as with our LiveAddress service. Once the address has been properly pinpointed and verified, the county is matched and returned.