There are 44 counties in Idaho. As of 2019, Idaho's most populous county is Ada County, home of Boise, with 469,966 residents. This represents a population growth of 19.48% since the 2010 census. Ada County is followed by Canyon County (223,499), Kootenai County (161,505), and Bonneville County (116,854). These are the only counties in Idaho with a population exceeding 100,000. Of these counties, Ada County experienced the highest population growth.
The least populated county in Idaho is Clark County with a population of 852 in 2019, down 13.06% since the last census. There are 15 counties in Idaho with fewer than 10,000 residents. Other small counties in Idaho are Camas County (1,127), Butte County (2,611), Lewis County (3,861), Adams County (4,250), Custer County (4,280), and Oneida County (4,488). Of these counties, Clark County experienced the most dramatic decline with a population growth rate of -13.06%. Adams County experienced the highest growth with a population that grew by 7.27% since the census.
Ada County, the largest county in Idaho, also experienced the most rapid growth over the last decade, climbing 19.48% to 469,966 people. Other cities that have experienced rapid growth include Canyon County (18.03%), Kootenai County (16.31%), Teton County (14.67%), Valley County (12.80%), and Jefferson County (12.27%). Only 8 counties in Idaho experienced negative growth since the census. The most dramatic decline was in Clark County, which dropped 13.06%, followed by Butte County at -10.43%. No other county experienced decline of more than 2%.