Fairfield, IA has a population of 9,942 and is the 3,951st largest city in the United States. The population density is 1,553 per sq mi which is 2743% higher than the Iowa average and 1614% higher than the national average. The median age in Fairfield is 37 which is approximately 2% lower than the Iowa average of 38. In Fairfield, 46% of the population over 15 years of age are married, 86% speak English and 2% speak Spanish. 43% of Fairfield residents were born in Iowa, 41% were born out of state, 1% were born outside of the United States and 15% were foreign born.