Boulder is a home rule city and the county seat of Boulder County, Colorado, United States. It is the largest city in Boulder County, with a population of 108,250 at the 2020 United States census, making it the 12th-most populous city in Colorado. Boulder is the principal city of the Boulder, CO Metropolitan Statistical Area and part of the Front Range Urban Corridor. Boulder is located at the base of the foothills of the Rocky Mountains, at an elevation of 5,430 feet above sea level.