Plainfield is a town in Sullivan County, New Hampshire, United States. At the 2010 census, the town had a total population of 2,364. The town is home to the Helen Woodruff Smith Bird Sanctuary and Annie Duncan State Forest.
According to the United States Census Bureau, the town has a total area of 52.9 square miles (137 km), of which 52.2 sq mi (135 km) is land and 0.8 sq mi (2.1 km) is water, comprising 1.42% of the town. Bounded on the west by the Connecticut River, Plainfield is drained by Blood’s Brook and Blow-me-down Brook. Croydon Mountain is a high ridge that occupies the eastern edge of the town. Grantham Mountain, a peak along the ridge, is the highest point in Plainfield at 2,660 feet (810 m) above sea level.
The town is crossed by the north-south state routes 12A and 120. Route 12A, serving the western part of the town, passes through the village of Plainfield. Route 120, passing through the eastern part of the town, serves the village of Meriden.
As of the census of 2010, there were 2,364 people, 923 households, and 684 families residing in the town. There were 984 housing units, of which 61, or 6.2%, were vacant. The racial makeup of the town was 98.0% white, 0.5% African American, 0.1% Native American, 0.5% Asian, 0.0% Native Hawaiian or Pacific Islander, 0.04% some other race, and 0.9% from two or more races. 1.1% of the population were Hispanic or Latino of any race.
Of the 923 households, 32.3% had children under the age of 18 living with them, 63.5% were headed by married couples living together, 7.0% had a female householder with no husband present, and 25.9% were non-families. 19.5% of all households were made up of individuals, and 6.4% were someone living alone who was 65 years of age or older. The average household size was 2.51, and the average family size was 2.88.
In the town, 22.8% of the population were under the age of 18, 4.4% were from 18 to 24, 22.4% from 25 to 44, 37.1% from 45 to 64, and 13.3% were 65 years of age or older. The median age was 45.2 years. For every 100 females, there were 100.0 males. For every 100 females age 18 and over, there were 97.5 males.
For the period 2011–2015, the estimated median annual income for a household was $82,250, and the median income for a family was $90,729. Male full-time workers had a median income of $56,385 versus $52,813 for females. The per capita income for the town was $38,810. 1.0% of the population and 0.3% of families were below the poverty line. 0.0% of the population under the age of 18 and 2.5% of those 65 or older were living in poverty.