There are 19 US Post Offices in Will County serving a total of 408,288 residents equating to 21,489 residents per post office. It's estimated that approximately 651,673 packages are handled across the 19 county post offices annually. Will County is a county located in the northern part of the state of Illinois. According to the 2010 census, it had a population of 677,560, which is an increase of 34.9% from 502,266 in 2000, making it the fourth-most populous county in Illinois. The county seat is Joliet. Will County is one of the five collar counties of the Chicago-Naperville-Elgin, IL-IN-WI Metropolitan Statistical Area. The portion of Will County around Joliet uses the 815 and 779 area codes, 630 and 331 area code for far northern Will County, and 708 area code for eastern Will County."