Planner Speak: What do you mean by density?

Your elementary school science teacher probably taught you about density. She might have shown you that an orange floats in water, but then peeled it, put it back in the water, and demonstrated how it sinks. She then explained that an orange rind is full of tiny air pockets that help to lower the density of an orange and allow it to float. “Density,” she explained, “is the mass of an object relative to its volume.” When planners talk about density in terms of land use, they have another definition altogether.

Density (n.) is the number of dwelling units per unit of area, such as 30 dwelling units per acre. A dwelling unit is the living quarters for a household, or one or more people who live together.  An apartment, house, townhome, and one side of a duplex are all examples of a dwelling unit. Calculating the density of an area is simple. Just divide the number of housing units by the number of acres.

Housing developments that have the same density can look very different as you can see in the images below. As Julie Campoli explained in her book Visualizing Denisty, “It is not density that makes a neighborhood appealing or appalling, but form – the street layout, arrangement of buildings, quality of architecture, and use of open space.” These are the factors that make a neighborhood feel crowded or spacious. Julie explains that people often assume dense neighborhoods have too many people, may not offer enough privacy, and the homes may be too close together; however, “how we perceive density has everything to do with how it is designed, not the actual ratio of units to acres.” To test your preconceptions of density, you can take this short quiz.

Last year Julie gave a City Share presentation in Chattanooga called Rethinking DensityYou can also learn more about density by visiting the Lincoln Institute of Land Policy site.


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Images Courtesy of the Commonwealth of Massachusetts

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