Illinois Counties by Population (2025)

There are 102 counties in Illinois.

The most highly populated county within the state of Illinois is Cook County, home to the city of Chicago. With a population of 5,180,493, Cook County has seen a decrease in the total population of -0.36% since the last U.S. census was taken in 2010.

Following Cook County, other large counties in Illinois include:

• DuPage County: 928,589
• Lake County: 700,832
• Will County: 692,310
• Kane County: 534,216 (growth rate of 3.50%)

Illinois Counties with the Fewest Residents

The county with the lowest population in the state of Illinois is Hardin County, which has a population of 3,910. Since the last census in 2010, Hardin County’s population has seen a decrease of -9.35%.

Other small counties in Illinois include:

• Pope County: 4,212
• Calhoun County: 4,802
• Scott County: 4,926

Illinois has 72 counties with a population of less than 50,000; all but one (Monroe County) experienced a decline in population. Of the 15 counties with fewer than 10,000 residents, Alexander County experienced the most dramatic decrease with a growth rate of -26.15%.

Illinois Counties with Rapid Growth

Kendall County is the fastest-growing county in Illinois with a population growth rate of 10.87%, reaching 127,915 residents.

Following Kendall County, other counties with notable growth include:

• Champaign County: Growth rate of 4.19%
• Monroe County: Growth rate of 4.05%
• Kane County: Growth rate of 3.50%

Only five other counties in Illinois had any increase in population since 2010. The county with the lowest growth rate in Illinois is Alexander County, which has seen a population decrease of -26.15%.