Population in dwellings density is the number of people in a chosen dwelling type per square kilometre of land. Place Forecast works it out as the resident population in dwellings divided by the land area.
This is a gross density measure. The numerator is one dwelling type's population. The denominator is the whole area's land, including roads, parks, and parts where that dwelling type does not exist.
Density is not the same as crowding. Crowding is about how many people share a home. Density is about how people spread over land.
Only the population in dwellings changes over time. The land area stays fixed.