Start with a declared numerator and denominator
Population density is population divided by area. If population is expressed as people and area as square kilometres, the result is people per square kilometre. For square miles, convert the area first or keep both quantities in the same unit system. Atlas Compare labels density as calculated because the displayed value is derived from the cited population and land-area inputs, not copied from a third independent source.
The inputs still need definitions. The World Bank describes its Population, total series as midyear population estimates and notes that source concepts and estimation methods can vary. Its Land area series excludes inland water bodies, continental-shelf claims, and exclusive economic zones. Read the linked definitions before treating two quotients as fully equivalent: World Bank population metadata and World Bank land-area metadata.
What the average does not describe
A whole-area density is an average across the selected boundary. It does not show where residents live inside that boundary, how much land is built up, or how population is distributed between neighbourhoods. A large low-density region can contain a compact city; one average cannot describe both patterns.
Density also does not measure housing availability, travel time, school performance, safety, or personal suitability. Those questions require different measures and carefully named geographies. Use the number as a geographic ratio, not as a quality score or a recommendation.
Keep dates and boundaries in view
Population and area can refer to different reference periods. In the U.S., the Census Bureau's GEOINFO tables report land and water area for covered geographies, while population sources have their own estimate or census dates. The Bureau explains the area fields in its land-area guidance. A boundary vintage is not the same thing as a population reference year.
Atlas Compare keeps source years beside the inputs and identifies density as a calculation. Where the dates or definitions differ, the quotient should not be described as a same-date official density statistic. See the guide to why population data years differ and the full sources and calculation method.
Recreate the calculation
To reproduce a result, note the population value and its reference year, the land-area value and its definition or boundary vintage, and the units used. Then divide population by area after any required unit conversion. The population-density calculator handles common area units in your browser. You can compare sourced state and country values in the comparison tool, or browse the individual state profiles and country profiles.