Fairfax, IA has a population of 2,418 and is the 10,021st largest city in the United States. The population density is 1,254 per sq mi which is 2196% higher than the Iowa average and 1284% higher than the national average. The median age in Fairfax is 37 which is approximately 3% lower than the Iowa average of 38. In Fairfax, 71% of the population over 15 years of age are married, 96% speak English and 3% speak Spanish. 82% of Fairfax residents were born in Iowa, 15% were born out of state, 0% were born outside of the United States and 3% were foreign born.