Population density is a measurement of population per unit area or unit volume, it is a quantity of type number density. It is frequently applied to living organisms, and most of the time to humans and it is a key geographical term. Population density is population divided by land area or water volume. Low densities may cause a vortex and lead to further reduced fertility. This is called the Allee effect after the scientist who identified it, commonly this may be calculated for a county, city, country, another territory, or the entire world. The worlds population is around 7,000,000,000, therefore, the worldwide human population density is around 7,000,000,000 ÷510,000,000 =13.7 per km2. If only the Earths land area of 150,000,000 km2 is taken into account and this includes all continental and island land area, including Antarctica. If Antarctica is also excluded, then population density rises to over 50 people per km2, thus, this number by itself does not give any helpful measurement of human population density. Several of the most densely populated territories in the world are city-states, microstates, cities with high population densities are, by some, considered to be overpopulated, though this will depend on factors like quality of housing and infrastructure and access to resources. Most of the most densely populated cities are in Southeast Asia, though Cairo, for instance, Milwaukee has a greater population density when just the inner city is measured, and the surrounding suburbs excluded. Arithmetic density, The total number of people / area of land, physiological density, The total population / area of arable land. Agricultural density, The total rural population / area of arable land, residential density, The number of people living in an urban area / area of residential land. Urban density, The number of people inhabiting an urban area / total area of urban land, ecological optimum, The density of population that can be supported by the natural resources. S. States by population density Selected Current and Historic City, Ward & Neighborhood Density
Population density (people per km2) map of the world in 1994
Monaco in Southern Europe, currently holds the record for being the most densely populated nation in the world.
Mongolia is the least densely populated country in the world.
This population cartogram of the European Union (2007-2012) uses areas and colors to represent population.