Polk City, IA has a population of 4,115 and is the 7,319th largest city in the United States. The population density is 808 per sq mi which is 1379% higher than the Iowa average and 792% higher than the national average. The median age in Polk City is 38 which is approximately 1% lower than the Iowa average of 38. In Polk City, 67% of the population over 15 years of age are married, 97% speak English and 3% speak Spanish. 73% of Polk City residents were born in Iowa, 26% were born out of state, 0% were born outside of the United States and 1% were foreign born.