Plainfield, PA has a population of 401 and is the 20,866th largest city in the United States. The population density is 621 per sq mi which is 124% higher than the Pennsylvania average and 585% higher than the national average. The median age in Plainfield is 35 which is approximately 14% lower than the Pennsylvania average of 41. In Plainfield, 44% of the population over 15 years of age are married, 100% speak English and 0% speak Spanish. 67% of Plainfield residents were born in Pennsylvania, 33% were born out of state, 0% were born outside of the United States and 0% were foreign born.