Carlstadt is a borough in Bergen County, New Jersey, United States. As of the 2010 United States Census, the borough's population was 6,127, reflecting an increase of 210 (+3.5%) from the 5,917 counted in the 2000 Census, which had in turn increased by 407 (+7.4%) from the 5,510 counted in the 1990 Census.
Carlstadt was originally formed as a village by an act of the New Jersey Legislature on March 12, 1860, within Lodi Township. Most sources indicate that the community was named for Dr. Carl Klein, the leader of a group of early German settlers who led the project to establish the community though Henry Gannett stated that the name derived from the city of Karlovac in Croatia, which was known as "Carlstadt" in German. The Borough of Carlstadt was incorporated on June 27, 1894, formally set off from Bergen Township. The borough was formed during the "Boroughitis" phenomenon then sweeping through Bergen County, in which 26 boroughs were formed in the county in 1894 alone.
According to the United States Census Bureau, the borough had a total area of 4.243 square miles (10.989 km2), including 3.999 square miles (10.358 km2) of land and 0.244 square miles (0.632 km2) of water (5.75%).
Carlstadt is bordered on the south by East Rutherford in Bergen County, Secaucus and North Bergen in Hudson County, on the north by Wood-Ridge and Moonachie (Bergen) to the east by Ridgefield and South Hackensack (Bergen), and to the northwest by Wallington (Bergen). The borough is approximately 8 miles (13 km) northwest of New York City and 95 miles (153 km) northeast of Philadelphia.
The 2010 United States Census counted 6,127 people, 2,378 households, and 1,578.992 families in the borough. The population density was 1,532.1 per square mile (591.5/km2). There were 2,495 housing units at an average density of 623.9 per square mile (240.9/km2).