Population density (in agriculture: standing stock or plant density) is a measurement of population per unit area, or exceptionally unit volume; it is a quantity of type number density. It is frequently applied to living organisms, most of the time to humans. It is a key geographical term.  In simple terms, population density refers to the number of people living in an area per square kilometre.
Population density is population divided by total land area or water volume, as appropriate. 
Low densities may cause an extinction vortex and lead to further reduced fertility. This is called the Allee effect after the scientist who identified it. Examples of the causes of reduced fertility in low population densities are: 
- Increased problems with locating sexual mates
- Increased inbreeding
Population density is the number of people per unit of area, usually quoted per square kilometre or square mile, and which may include or exclude for example areas of water or glaciers. Commonly this may be calculated for a county, city, country, another territory or the entire world.
The world's population is around 7,500,000,000  and Earth's total area (including land and water) is 510,000,000 km2 (197,000,000 sq. mi.).  Therefore, the worldwide human population density is around 7,500,000,000 ÷ 510,000,000 = 14.7 per km2 (38 per sq. mi.). If only the Earth's land area of 150,000,000 km2 (58,000,000 sq. mi.) is taken into account, then human population density is 50 per km2 (129 per sq. mi.). This includes all continental and island land area, including Antarctica. If Antarctica is also excluded, then population density rises to over 55 people per km2 (over 142 per sq. mi.).  However, over half[ citation needed] of the Earth's land mass consists of areas inhospitable to human habitation, such as deserts and high mountains, and population tends to cluster around seaports and fresh-water sources. Thus, additional criteria are needed to make simple population density values useful.
Several of the most densely populated territories in the world are city-states, microstates and urban dependencies.   In fact, 95% of the world's population is concentrated on just 10% of the world's land.  These territories have a relatively small area and a high urbanization level, with an economically specialized city population drawing also on rural resources outside the area, illustrating the difference between high population density and overpopulation.
Deserts have very limited potential for growing crops as there is not enough rain to support them. Thus their population density is generally low. However some cities in the Middle East, such as Dubai, have been increasing in population and infrastructure growth at a fast pace. 
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.  Very densely populated cities are mostly in Asia (particularly Southeast Asia); Africa's Lagos, Kinshasa and Cairo; South America's Bogotá, Lima and São Paulo; and Mexico City and Saint Petersburg also fall into this category. 
City population and especially area are, however, heavily dependent on the definition of "urban area" used: densities are almost invariably higher for the centre only than when suburban settlements and intervening rural areas are included, as in the agglomeration or metropolitan area (the latter sometimes including neighboring cities).
In comparison, based on a world population of 7 billion, the world's inhabitants, as a loose crowd taking up almost 1 m2 (10 sq. ft) per person (cf. Jacobs Method), would occupy a space a little larger than Delaware's land area.[ citation needed]
|km2||sq. mi.||per km2||per sq.|
|1||Macau ( China)||30.5||12||650,834||21,339||55,268|
|4||Hong Kong ( China)||1,106.3||427||7,409,800||6,698||17,348|
|5||Gibraltar ( UK) ||6.8||2.6||33,140||4,874||12,624|
|10||Bermuda ( UK)||52||20||63,779||1,227||3,178|
|km2||sq. mi.||per km2||per sq.|
Although arithmetic density is the most common way of measuring population density, several other methods have been developed to provide a more accurate measure of population density over a specific area.
- 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
- Distance sampling
- Human geography
- Idealized population
- List of population concern organizations
- Optimum population
- Plant density
- Population dynamics
- Population genetics
- Population health
- Population momentum
- Population pyramid
- Rural transport problem
- Small population size
- List of Australian suburbs by population density
- List of countries by population density
- List of cities by population density
- List of city districts by population density
- List of English districts by population density
- List of European cities proper by population density
- List of islands by population density
- List of U.S. states by population density
- List of United States cities by population density
- Matt Rosenberg Population Density. Geography.about.com. March 2, 2011. Retrieved on December 10, 2011.
- Minimum viable population size. Archived October 6, 2012, at the Wayback Machine Eoearth.org (March 6, 2010). Retrieved on December 10, 2011.
- U.S. & World Population Clocks. Census.gov. Retrieved on December 10, 2011.
- World. CIA World Handbook
- Department of Economic and Social Affairs Population Division (2009).
"World Population Prospects, Table A.1" (PDF). 2008 revision. United Nations. Retrieved March 12, 2009. Cite journal requires
- The Monaco government uses a smaller surface area figure resulting in a population density of 18,078 per km2
- "Urbanization: 95% Of The World's Population Lives On 10% Of The Land". ScienceDaily. Retrieved September 1, 2020.
- Portnov, B. A.; Hare, A. Paul (1999). Desert regions : population, migration, and environment. Springer. ISBN 3540657800. OCLC 41320143.
- Human Population. Global Issues. Retrieved on December 10, 2011.
- The largest cities in the world by land area, population and density Archived May 16, 2015, at the Wayback Machine. Citymayors.com. Retrieved on December 10, 2011.
- Territory claimed by Spain.
- Selected Current and Historic City, Ward & Neighborhood Density
- Duncan Smith / UCL Centre for Advanced Spatial Analysis. "World Population Density".