Is a website about density. Use it as a reference for understanding patterns in the American built and social environment at the census tract level.

To put Census data in context, this site displayed statistics for a tract relative to the number of people who live in tracts with lower values. These are the middle tracts for some of the data points tracked on the site. Compared to the values for each statistic here, half of the US population lives in tracts with lower values, half in tracts with higher values.

The site includes data for the areas that the Census collects the most detailed data: the fifty states, the District of Columbia and Puerto Rico.


Some entries adapted from the US Census glossary.

Census tract
A small, subdivision of a county created to track statistics. Census tracts ideally contain about 4,000 people and 1,600 housing units.
Paid employment consists of full and part-time employees. To calculate the number of employees within 30 miles, census tracts where compared to nearby zip codes. The radius of 30 miles was chosen as a reasonable number on the high end for commuting distance.
Housing unit

A place where people live, whether a house, an apartment, a mobile home or trailer, a group of rooms, or a single room occupied as separate living quarters. Also includes vacant places intended for occupancy as separate living quarters. Separate living quarters are those in which the occupants live separately from any other individuals in the building and which have direct access from outside the building or through a common hall. For vacant units, the criteria of separateness and direct access are applied to the intended occupants whenever possible.

Housing unit density

The number of housing units within a geographic area (e.g, the United States, a city), divided by the land area. Housing unit density is commonly as either "housing units per square kilometer", “housing units per square mile" or "housing units per acre".

Population density

The total population within a geographic area (e.g, the United States, a city) divided by the land area. Population density is commonly expressed as "people per square kilometer" or "people per square mile".

Single-Family House

Single-family houses may be fully detached, semi-detached (side-by-side), row houses, or townhouses. In the case of attached units, each must be separated from the adjacent unit by a ground-to-roof wall in order to be classified as a single-family structure. Also, these units must not share heating/air-conditioning systems or utilities, such as water supply, power supply, or sewage disposal lines.

Single-family statistics do not include units built one on top of another, built side-by-side, that lack a ground-to-roof wall, nor those that share space or utilities (e.g, an attic or plumbing pipes).

Median income

Half of households take in an income more than the median income, half less. A median is calculated by sorting a group by a value, and then finding the value at the middle.

Owner-occupied housing unit

A housing unit is owner-occupied if the owner or co-owner lives in the unit, even if it is mortgaged or not fully paid for.

Year structure built

The year a building was first constructed, not when it was remodeled, added to, or converted. This data doesn't count buildings that have been torn down.

Vacancy status

Unoccupied housing units are considered vacant. A unit may be vacant status because it is for rent or sale, or because it's only used seasonally.


Packages created in the course of making this project:

Many thanks to the creators and maintainers of Colorbrewer, csvkit, d3, GDAL, JQ, J, Leaflet, Mapshaper, Python, SQLite, Spatialite.

© 2017 Neil Freeman