ZIP Codes - Tools & Resources from the Urban Information Center

The ZIP Code Resources Page

Tools and Resources Related to U.S. ZIP Codes

This page describes a series of tools for helping users deal with 5-digit U.S. postal ZIP code areas. We have focused primarily on tools for linking ZIP codes to other geographies (such as counties, cities, metro areas) and to demographic information from the 1990 decennial census. Most of these resources have been publicized elsewhere but this page is an attempt to consolidate everything in a single place.

General Information | Geographic Reference Tools | Demographic Profiles | Links to Other Pages

General Information About ZIP Codes

Problems With Spatial Definition

ZIP codes are a very messy kind of geography. They were created by the U.S. Postal Service as a tool to help deliver the mail more efficiently. ("ZIP" is actually an acronym for "Zone Improvement Program", a reference to the 2-digit postal zones that were used by the post office prior to implementing nationwide ZIP codes back in the early 1960's. Because it is an acronym we always use the uppercase for it.) ZIP codes have been adopted by marketing people and by all kinds of other researchers as a standard geographic area, like a city or a county. We see maps of ZIP codes in telephone books and from commercial vendors that make us think of them as spatially defined areas with precise boundaries, similar to counties. But, from the perspective of the post office, ZIP codes are not and never have been such spatial entities. They are simply categories for grouping mailing addresses. As such, ZIP codes do in most cases resemble spatial areas since they are comprised of spatially clustered street ranges. But not always. In rural areas, ZIP codes can be collections of lines (rural delivery routes) that in reality do no look much like a closed spatial area. In areas where there is no mail delivery (deserts, mountains, lakes, most of Nevada) ZIP codes are really not defined. You may see maps that show ZIP code boundaries that include such areas, but these are not post-office-defined official definitions. They will not be assigned a code until there is a reason for it, i.e. until there needs to be mail delivered there. What this means is that the actual defintion of a ZIP code "boundary" is quite fuzzy at best, and a purely extrapolated guess (at what it would be if someone were to start receiving mail there) at worst. If you have an application that requires extreme geographic precision then you need to avoid using ZIP codes.

The Time Dimension: Current Versus 1991 ZIP Codes

An important thing to keep in mind about ZIP codes is that they change over time. In some cases these changes can be quite dramatic, but more commonly they are small and subtle. When a ZIP codes changes its definition it does not change its name like a census tract. The ZIP code that was called '63301' in St. Charles county, Mo in 1985 has since been broken into first two and now three ZIP codes. These new codes were not called 63301.01, 63301.02 and 63301.03; they were called 63301, 63303 and 63304. So what is referred to as 63301 today represents about a third of the area that it referred to in 1985. The new code 63303 did not exist 12 years ago and it has already changed its definition so that it now represents about half of the area it included when it was intitially created. What this means, of course, is that ZIP codes are really terrible units for doing any kind of time-series analysis unless you have some way of keeping track of all the changes over time. Otherwise, you may wind up concluding that there has been a dramatic downward trend in the population of 63301 since 1980, when in fact just the opposite is true.

What the world really needs to deal with ZIP code geography properly is a large geographic equivalency file relating ZIP codes to other relevant geographies with a time dimension. Instead, what we have is such an equivalency file that relates ZIP codes to geographic entities primarily used for taking the 1990 census. This file uses ZIP codes as they were defined around July of 1991. (Because it takes a long time to do the research, it may be that the currency of the ZIP codes used varied somewhat from area to area.) What we are referring to here is the MABLE database and the corresponding Geocorr web application which we'll be talking about in more detail below. For now, what we want to emphasize is that when we talk about ZIP codes we really need to keep a time reference in mind. Just as when you work with census tracts you need to know whether you mean 1980 or 1990 tracts, or when you are talking about the countries of Europe -- time is an important dimension.

You might think that what we should always assume is that if we do not specify a time, then we are probably referring to the most current definitions of ZIP codes, and that any reference materials should be periodically updated to reflect these definitions. Easier said than done, of course. This would be a huge task. But even if you could maintain all your lists with the latest definitions in some cases there are reasons why it may be preferable not to. This has to do with the fact that the Census Bureau tabulated the results of the 1990 census to produce a file called "Summary Tape File 3B" (aka "STF3B"). The data tables on that file describe the characteristics of the residential ZIP codes as they were (more or less) defined at the time the special tabulations were prepared (circa July, 1991.) "More or less"? Yes - for the purposes of creating these tabulations the Bureau had private vendors provide them with files that related each of the 1990 census blocks (the smallest geographic unit identified for each 1990 census return) with the current ZIP code definitions ("current" as of that point in time, of course.) The files created by these vendors were used to create a data product called the "ZIP Block Equivalency Files" or "STF3B Headers Files". They define which geographic areas were used to approximate the ZIP code areas being summarized by the STF3B data tables. There is a builtin "fuzz factor" in this equivalency list since - while the Census Bureau has created census blocks so that they do not cross any other census-defined geographic unit - blocks can and do (frequently) cross ZIP codes. Typically, ZIP code "boundaries" fall along back lot lines - they almost never split down the middle of a street. If they did, you would need to have two postal carriers - one from each of the two ZIP codes, travel the same street and deliver just to their side. Census blocks, however, almost always split down the middle of streets. As a result, blocks near the boundaries of ZIP codes typically split ZIP codes, with one side of the street in one ZIP and the other side in a diferrent ZIP. Yet, in the block to ZIP equivalency file prepared by the vendors each block was assigned to one and only one census block.
These ZIP Equivalency Files were the basis of the Master Area Block Level Equivalency ("MABLE") file used in the MABLE/Geocorr web application.

Most of the various files that we reference from this page will be dealing with ZIP codes as they were defined for the purposes of tabulating the 1990 census ZIP code summaries. In other words, the "default time" for ZIP codes for the sake of this document is approximately July of 1991.

Failure to Conform to Other Geographic Schemes

Another important and exasperating characteristic of ZIP codes is that they do not conform to any other geographic schemes. Most geographic units are part of some hierarchal system, and frequently they will recognize other boundaries such as counties or states. But ZIP codes follow no rules whatsoever with respect to other geographies. ZIP codes can and do cross state lines (rarely, but enough to cause problems and confusion), county lines (about 10% of ZIPs are in more than one county), political jurisdictions (cities, congressional districts), metro areas, etc.

This aspect of ZIPs and several other useful bits of information about them are discussed in the MAGGOT (Master Area Geographic Glossary of Terms) file provided with MABLE/Geocorr.

ZIP Code Geographic Reference Tools

Ready to Read Reference Tables

We have created a series of tables which define the correspondence between 1991 ZIP codes and other commonly used geographic areas. The typical use of such files is in processing files which contain ZIP codes. It is often helpful to be able to place an address into a county or a metropolitan area when all you have to go on is the ZIP code.

These "tables" are ascii files that can be easily browsed and downloaded. But they are also in a special form for use within the SAS(r) software package. SAS has a feature that allows creating table-lookup data entities called SAS formats. If you are a SAS programmer you use these modules to create such lookup tables which will let you use the SAS put function to directly convert ZIP codes to these other geographic codes.

Caution: Some of these files are quite large and could cause problems if you do not have enough memory available.

ZIP to state: relates ZIPs to state postal abbreviations.
ZIP to county: (774K) relates ZIPs to primary county (the one in which it is contained or with which it has the largest intersection). Shows the county FIPS code and name.
ZIP to metro area: relates ZIPs to primary metro area (the one in which it is contained or with which it has the largest intersection). Shows the FIPS code and name of the metro area (using June 1995 metro area definitions).
ZIP names: (2.9M) Provides a geographic name for each ZIP (hybrid source, mostly USPS (U.S. Postal Service) - note large size).
ZIP to many codes: (2.4M) This is a very large lookup table that returns a whole series of geographic codes for each ZIP. It has been formatted in a way that makes it easy to read by a program because the codes appear in fixed columns of the record. See next entry.
Usage notes for previous entry: This entry provides detailed documentation for the previous ZIP to many codes entry.
Tips and usage notes suggesting some strategies for using the other modules for encoding your address files. Adapted from messages posted by John Blodgett to SAS-L listserv. Directly intended for SAS users but possibly could be adapted for other packages.
ZIP Land Areas and Population Centroids: This comma-separated-value file contains one line for each of the ZIP codes in the MABLE data base. We aggregated the land area of all census blocks associated with the ZIP code, and then took a population-weighted average of the internal block latitude-longitude coordinates for each census block in order to create the "population centroid" coordinates appearing on this file.

Dynamically Generated Correspondence Files Using Geocorr

The above section is useful for accessing some of the most commonly-used geographic correspondences between ZIPs and other geographic units. But there are lots of other possibilities. Like relating ZIPs to census tracts, congressional districts, 10-mile radius of the proposed atomic waste dump site, etc. These lists can be generated using the MABLE/Geocorr geographic correspondence engine. It can also be used just to get lists of ZIP codes and 1990 population counts for anywhere in the U.S.

Note: MABLE/Geocorr was developed by SEDAC (Social and Economic Data Access Center) at CIESIN (Consortium for International Earth Sciences Information Network) with the bulk of the programming work done under contract by John Blodgett, then of the University of Missouri St. Louis.

Recent ZIP code names and County/Metro Codes via MSCDC's uexplore/xtract

The Missouri Census Data Center's web-accessible data archive can be "explored" using the uexplore cgi-bin application. Using this directory navigating tool, you can access metadata and then the data itself using the hypercon and xtract applications. One of the files (SAS datasets, actually) on the archive has data about all current US ZIP codes and includes primary FIPS state and county codes, plus lists of place names associated with each ZIP. This is all derived from the U.S. Postal Service's City State file. Not everyone can easily use these extraction tools (it helps of you are comfortable with SQL queries), but if you click here you will be dropped into a uexplore session to view the main zipcit directory. A number of html files can be viewed prior to clicking on the us96.ssd01 file, and then choosing to do an extaction. It will take a little clicking but it will let you create your own custom extract/report in any of 5 formats for any geographic areas you specify. You may want to have your FIPS codes handy if you want to use them to filter your data.

Demographic Profiles of ZIP Codes

This application lets you enter a ZIP code and get a brief demographic summary of that area as derived (by the Missouri Census Data Center) from the above-mentioned 1990 STF3B tabulations. (Enter the codes in the boxes and click on "Submit Request").
Note: If the ZIP code did not exist in 1991 then there will not be a profile for it. If it existed then but has changed its definition since, the profile is for the old (1991) definition.

Note: There is also a web application that lets you do this (generate ZIP profiles) with a menu-driven interface (not just to ZIP codes but to most other popular geographic levels as well.)

The Missouri Census Data Center maintains a complete collection of census data for all U.S. ZIP codes for 1980 and 1990. We can provide custom programming services related to these data.

Links to Other ZIP-related Resources

Deane Merrill's ZIP page has all kinds of goodies related to ZIP and other postal geography.
The U.S. Postal Service's ZIP Code Lookup and Address Information Page is the ultimate authority for current ZIP geography.
Free ZIP code data for any ZIP (latest estimates from CACI). It's a teaser page to get you to buy the product line, of course. There are many data vendors selling updated and enhanced demographic information for ZIP codes. Just try your favorite web search engine using "zip*code*demographic" to see much of what's available.
ZIP Links from Bridger Systems is another page that offers free samples of data and software that it wants to sell. It lets you enter any ZIP code in the country and a radius in miles, and have returned to you a report showing all ZIP codes within the specified radius, along with a current population estimate for each ZIP (with a total of these as well). Other options let you calculate the distance between 2 ZIP codes; and to enter a city name and get the ZIP code(s) associated with it.

This page maintained by:: John Blodgett blodgettj@umsystem.edu; Office of Social and Economic Data Analysis (OSEDA); (Formerly with the Urban Information Center, U of Missouri St. Louis); 626 Clark Hall / University of Missouri / Columbia, MO 65211

John Blodgett's Home Page | Missouri Census Data Center | OSEDA

URL: http://oseda.missouri.edu/uic/ZIP.resources.html