Northfield is a town in Merrimack County, New Hampshire, United States. The population was 4,829 at the 2010 census.
According to the United States Census Bureau, the town has a total area of 29.0 square miles (75.1 km), of which 28.8 sq mi (74.6 km) is land and 0.3 sq mi (0.8 km) is water, comprising 0.89% of the town. It is drained by the Merrimack River, primarily by way of the Winnipesaukee River and its tributary Williams Brook. The town is characterized by gently rolling hills, the highest point in town being the summit of Bean Hill at 1,500 feet (460 m) above sea level, near the southern border. Two significant water bodies in Northfield are Sandogardy Pond in the southwest corner of town and Knowles Pond on the east side of town.
The village of Northfield is located in the northern part of the town on the south bank of the Winnipesaukee River. Directly across the river is the village of Tilton in Belknap County. The two villages together comprise the Tilton Northfield census-designated place.
As of the census of 2010, there were 4,829 people, 1,843 households, and 1,309 families residing in the town. There were 1,969 housing units, of which 126, or 6.4%, were vacant. The racial makeup of the town was 96.6% white, 0.8% African American, 0.3% Native American, 0.8% Asian, 0.0% Native Hawaiian or Pacific Islander, 0.2% some other race, and 1.3% from two or more races. 1.4% of the population were Hispanic or Latino of any race.
Of the 1,843 households, 36.2% had children under the age of 18 living with them, 52.9% were headed by married couples living together, 12.7% had a female householder with no husband present, and 29.0% were non-families. 20.7% of all households were made up of individuals, and 5.7% were someone living alone who was 65 years of age or older. The average household size was 2.60, and the average family size was 2.98.
In the town, 24.6% of the population were under the age of 18, 8.1% were from 18 to 24, 26.3% from 25 to 44, 31.9% from 45 to 64, and 9.1% were 65 years of age or older. The median age was 39.2 years. For every 100 females, there were 97.7 males. For every 100 females age 18 and over, there were 95.0 males.
For the period 2011-2015, the estimated median annual income for a household was $65,536, and the median income for a family was $72,138. Male full-time workers had a median income of $51,056 versus $40,184 for females. The per capita income for the town was $28,007. 6.7% of the population and 5.4% of families were below the poverty line. 12.9% of the population under the age of 18 and 1.3% of those 65 or older were living in poverty.