Planopedia

Density is the number of developed units in a specific area of land. Residential density, for example, is usually measured by dwelling units per acre (du/ac). The density of developments is determined by zoning codes, and can be an extremely contentious issue in planning and development debates at the local level. 

Many forms, and variable definitions, of density are common in discussions about planning and zoning, but the distinctions between different forms of density can be informative in their significance. While residential or development density is measured by dwelling units per acre, population density is measured by residents per acre, for example. Employment density—usually measured by jobs per acre—is an important consideration in commercial districts and office parks.  

Floor-to-area ratio (commonly referred to as FAR) is frequently referred to in the media as a metric for development density, but that definition is actually a mistake. FAR measures development intensity or building size, separated from the numerator of use (jobs or dwellings, for example) found in every definition of density. FAR is therefore subject to some variability in density. FAR only goes so far, in other words, in setting the stage for density.

Used as a shorthand in public conversations about planning and zoning, density usually implies a defining characteristic of urban settings, and thus also usually implies some variety of multi-family housing—apartments or condominiums. But it's important to note how much effect design can have on the perception of density, however. Seven dwelling units per acre, for example, can take many forms, and can be tailored to match their surrounding neighborhoods. In the politics of development, supporters of more suburban environments or single-family neighborhoods in cities usually view density as an undesirable goal, and urban density has been associated with negative environmental and societal consequences in the past, as well as in many parts of the contemporary world. 

Density has achieved political and cultural victories in the United States in recent years, as reforms like Missing Middle Housing, Smart Growth, and Accessory Dwelling Units have won converts to the benefits of density among planners, politicians, and the public. Those benefits—which are still hotly debated in academia and among the public—include efficiencies in energy consumption, mobility, and the economy, as a few examples. For many urban dwellers, density of urban living is a perk of city living.