Plainfield is a census-designated place in Cumberland County, Pennsylvania, United States. The population was 399 at the 2010 census. It is part of the Harrisburg–Carlisle Metropolitan Statistical Area.Plainfield is located in the eastern part of West Pennsboro Township at 40°12′7″N 77°17′12″W / 40.20194°N 77.28667°W / 40.20194; -77.28667, along Pennsylvania Route 641, which leads east 5 miles to the center of Carlisle, the county seat, and west 6 miles to Newville. The Pennsylvania Turnpike passes along the northern edge of the CDP, but the nearest exit is 8 miles to the east, on the other side of Carlisle.According to the United States Census Bureau, the Plainfield CDP has a total area of 0.64 square miles, all of it land.As of the census of 2000, there were 376 people, 141 households, and 109 families residing in the CDP. The population density was 594.3 people per square mile. There were 162 housing units at an average density of 256.1/sq mi. The racial makeup of the CDP was 98.