There are 56 counties in Montana. The most populous county located in the state of Montana is Yellowstone County, home to Billings, which has a population of 160,137 in 2019. Since the time that the last census was taken in 2010, Yellowstone County has seen an increase in the total population of 7.92%. The only other counties in the state of Montana with a population of more than 100,000 are Missoula County (118,791, Gallatin County (111,876), and Flathead County (102,106). All four counties have experienced substantial population growth of at least 7%; Gallatin County grew the most with a 24.79% growth rate.
The county with the lowest population in the state of Montana is Petroleum County, which has a population of 513 people. Since the last census in 2010, Petroleum County’s population has seen an increase of 4.91%. Following up after Petroleum County, other small counties in Montana include Treasure County (679), Golden Valley County (826), Wibaux County (1,034), and Prairie County (1,087). Of these counties, Petroleum County experienced the most significant growth while Prairie County had the most dramatic decline with a growth rate of -8.11%.
The county with the most significant growth rate in the state of Montana is Gallatin County, which has seen a population growth of 24.79% since the 2010 census was taken, bringing the population up to 111,876 in 2019. Montana has several other counties that have experienced double-digit growth since the census including Madison County (13.91%), Flathead County (12.37%), and Richland County (11.82%). Toole County has seen the lowest population growth, with a loss of 9.17%. )