Boulder, CO has a population of 105,420 and is the 288th largest city in the United States. The population density is 4,012 per sq mi which is 7958% higher than the Colorado average and 4328% higher than the national average. The median age in Boulder is 29 which is approximately 21% lower than the Colorado average of 36. In Boulder, 36% of the population over 15 years of age are married, 85% speak English and 7% speak Spanish. 27% of Boulder residents were born in Colorado, 61% were born out of state, 2% were born outside of the United States and 11% were foreign born.