Plainfield, IA has a population of 505 and is the 19,463rd largest city in the United States. The population density is 1,507 per sq mi which is 2658% higher than the Iowa average and 1563% higher than the national average. The median age in Plainfield is 37 which is approximately 2% lower than the Iowa average of 38. In Plainfield, 66% of the population over 15 years of age are married, 98% speak English and 1% speak Spanish. 84% of Plainfield residents were born in Iowa, 15% were born out of state, 1% were born outside of the United States and 0% were foreign born.