Flanders lies in the German Valley, between highlands that rise 400 feet (120m) to the northwest and 200 feet (60m) to the southeast. Drakes Brook, a southwest-flowing tributary of the South Branch of the Raritan River, passes through the center of town. According to the U.S. Census Bureau, the Flanders CDP has a total area of 7.45 square miles (19.30km2), of which 0.03 square miles (0.08km2), or 0.36%, are water.
As of the 2020 census, Flanders had a population of 9,832.
The median age was 40.2 years. 21.5% of residents were under the age of 18 and 14.2% of residents were 65 years of age or older. For every 100 females there were 97.1 males, and for every 100 females age 18 and over there were 97.4 males age 18 and over.
98.1% of residents lived in urban areas, while 1.9% lived in rural areas.
There were 3,792 households in Flanders, of which 32.1% had children under the age of 18 living in them. Of all households, 56.4% were married-couple households, 15.8% were households with a male householder and no spouse or partner present, and 21.1% were households with a female householder and no spouse or partner present. About 24.0% of all households were made up of individuals and 7.4% had someone living alone who was 65 years of age or older.
There were 3,967 housing units, of which 4.4% were vacant. The homeowner vacancy rate was 1.6% and the rental vacancy rate was 6.6%.
Flanders CDP, New Jersey – Racial and ethnic composition Note: the US Census treats Hispanic/Latino as an ethnic category. This table excludes Latinos from the racial categories and assigns them to a separate category. Hispanics/Latinos may be of any race.
↑Thompson, Rich. "Holy Cross LB’s Dobbs, Anderson committed to fourth straight PL title", Boston Herald, August 8, 2022. Accessed January 2, 2024. "On most FCS defenses, Anderson would anchor the front seven for his abilities as a tackler and a turnover machine. The 6-3, 225-pounder from Flanders, N.J., finished with 81 tackles, 15 tackles for a loss, 7.5 sacks with three interceptions and a forced fumble."
↑Spaulding, Anthony. "After breaking leg in 2015, former PJ star Noah Brown to start for Ohio State on Saturday"Archived January 16, 2017, at the Wayback Machine, New Jersey Herald, September 2, 2016. Accessed January 12, 2017. "Noah Brown has every reason to be pumped up for Saturday.That day at noon, the Flanders native and 2014 Pope John High School graduate is expected to start in the first NCAA Division I college football game of his career at wide receiver in the Ohio State Buckeyes' season opener against Bowling Green."