A census block is the smallest geographic unit used by the United States Census Bureau for tabulation of 100-percent data (data collected from all houses, rather than a sample of houses). Several blocks make up block groups, which again make up census tracts. Census blocks are areas bounded on all sides by visible features, such as streets, roads, streams, and railroad tracks, or by invisible boundaries, such as city, town, township, and county limits, property lines, and short, imaginary extensions of streets and roads. Generally, census blocks are small in area; for example, a block bounded by city streets. However, census blocks in remote areas may be large and irregular and contain many square miles . There are on average about 39 blocks per block group, but there are variations. Blocks typically have a four-digit number ranging from 0000 to 9999 within each census tract. The first number indicates which block group the block is in; for example, Block 3019 would be in block group 3. The number of blocks in the United States, including Puerto Rico, for the 2010 Census was 11,155,486 .
Most people intuitively think of census blocks as being rectangular or square, of about the same size, and occurring at regular intervals. In many cities of the United States, however, census block configurations actually are quite different. Patterns, sizes, and shapes of census blocks vary within and between areas. Factors that influence the overall configuration of census blocks include topography, the size and spacing of water features, the land survey system, and the extent, age, type, and density of urban and rural development . In cities a census block may correspond to a city block, but in rural areas where streets are fewer, may include several square miles and have some boundaries that are not streets . The population of a census block varies greatly. There are about 2,700,000 blocks with a population of 0 in the United States, while a block with an apartment complex may have several hundred inhabitants.
Census blocks are not:
- Based on population. However, Census Block Groups generally contain between 600 and 3,000 people .
- Permanent. Blocks can be split and moved if a change occurs to another geographic boundary. In the case of a split, the two new block numbers get a letter suffix added to them. For example, if block 3078 were to split, then the new blocks would become 3078A and 3078B.
- Usable with American Community Survey Data. Census Block data is too specific; the American Community Survey data only goes as far down as block groups. 
Census blocks covering the entire country were introduced with the 1990 census. Prior to this, back to the 1940 census, only select areas were divided into blocks.
Use in GIS
Statistical data from censuses are easily obtainable and useful for many applications, and are therefore frequently used in GIS projects. Census blocks are commonly used as districts on large-scale choropleth maps or aggregated into larger districts such as census tracts or counties. Censuses in the United States and many other countries are done every ten years. The United States federal government has made census data for the United States from recent decades available to be downloaded for free from the internet, including shape files for the census blocks, and aggregate statistical data for the blocks. Historical data from earlier United States censuses (which go back to 1790) which are usable in GIS have been prepared by private organizations.
- US Census Bureau
- http://www.census.gov/geo/www/GARM/Ch11GARM.pdf. U.S. Census Bureau. Retrieved October 21, 2012
- http://support.esri.com/en/knowledgebase/GISDictionary/term/census%20block. GIS Dictionary. Retrieved October 21, 2012
- Borden D. Dent, Jeffrey S. Torguson, Thomas W. Hodler, Cartography: Thematic Map Design, Sixth Edition (New York City: McGraw-Hill, 1999), 76.
- The National Historical Geographic Information System, https://www.nhgis.org/ (accessed 6 November 2017).