German population density map: https://commons.wikimedia.org/wiki/File:Population_density_in_Germany.png
U.S. population density map: https://es.m.wikipedia.org/wiki/Archivo:US_population_map.png
1 square mile is about 2.6 square km, so 2500 people in one square km is about 6500 people per square mile.
21 has been overestimated.
German population density map: https://commons.wikimedia.org/wiki/File:Population_density_in_Germany.png
U.S. population density map: https://es.m.wikipedia.org/wiki/Archivo:US_population_map.png
1 square mile is about 2.6 square km, so 2500 people in one square km is about 6500 people per square mile.