The cases in this atlas show three different quantitative measurements of density: dwelling units per hectare or acre, people per hectare or acre, and floor area ratio. Although these are common measures of density, they are often used alone, without respect to the other measures. It is important to look at all three numbers to obtain an accurate depiction of density.
The diagram below details the three measures.
Why these three measures? These are the three most commonly used measurements of density and each describes density from a different perspective. Each measurement looks at a different aspect of density and alone, each measurement is used to plan for a specific set of needs. Urban planners often are most concerned with FAR measures; realtors are often most concerned with dwelling units due to their focus on renting and selling; and government agencies care most about population numbers, which are tied to city services and infrastructure needs. Although each measurement provides good information about a place, alone, they do not paint a complete picture of the density of a neighborhood.
A better understanding of the density of a place comes not only from the additional information supplied from each of the three measurements, but also through looking at the three measurements relative to each other. For example, two areas may have the same number of dwelling units, but may feel more or less dense based on the relationship of those units to the number of people and the FAR in the area.
FAR + Coverage
Floor Area Ratio (FAR) is the ratio of built area (a building's total size) to the lot area (the property upon which the building is built). It is a measure used by planners, regulators, and developers to discern the intensity of a development. By itself, however, it is not sufficient to define density.
Coverage is the relationship between the ground floor area of enclosed buildings and the area of the lot. Development scenarios with the same FAR but different coverage will produce varying types of development: for example, low-rise or high-rise.
The examples below are a classic case of varying perceptions of density for two areas with identical FAR.
Another measure of density is the number of dwelling units built on the lot, often used by realtors or developers, as their focus is the marketable number of units in a given area. The density of an area can change based on the number of dwelling units – although we use individual dwelling units as a measurement, we don’t measure the size of the units. Large buildings may take up the same amount of space as small ones, resulting in similar levels of FAR. Is an area filled with McMansions as dense as an area filled with the same number of rowhouses?
Measuring the number of people in a given area is helpful to measure density, however, it does not measure the amount of living space per person. Are dwelling units a comfortable size? Is there public space for people? How many people live in each household?
It is clear that there are complications with using only one of these three measures to analyze the density of an area. When looking at cases, you will understand the importance of looking and comparing all three measures. For example, Battery Park City may feel dense, however, individual unit sizes are fairly large, with few people per household – a case of an area with high FAR, medium number of dwelling units medium to low population. Informal developments, by contrast, have low FAR, and a medium to high number of dwelling units and population, which can yield less square feet of living space per person.
Understanding these numbers will help city planners and others determine the best mix of these measurements for their neighborhood or town. Two urban areas with very similar density levels can be arranged in very different ways. How a planner or developer manages the urban design process and qualitative factors of an area is critical in developing an area appropriately to its cultural context.
And of course, the feel of a place is also dependent on the urban design features, lot coverage, open space, architecture and street design. These factors will contribute to the feel of a place but are independent from the quantitative measures of density. Our collection of case studies allows one to compare places to understand the similarities and differences caused by different levels of density. Two places may have the same quantitative measurement while looking very different.