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A postal carrier route is the group of addresses to which the USPS assigns the same code to aid in mail delivery. These codes are 9 digits – 5 numbers for the ZIP Code, one letter for the carrier route type (explained below), and 3 numbers for the carrier route code. For example "05055R003" or "12508C007." Typically, each carrier route is related to where a particular mail carrier delivers.

There are approximately 600,000 unique carrier routes in the US. Of these, approximately half are assigned to post-office boxes, leaving approximately 300,000 that represent “boundaries” or delivery areas. These numbers fluctuate from month to month. Since there are approximately 40,000 valid ZIP Codes in the US that translates to an average of 15 carrier routes per ZIP Code. Obviously any particular ZIP Code could have many more or many fewer than this average.

Yes. The USPS makes minor changes to the delivery routes continuously, usually due to either population or staff changes. For example, as new homes are built, those new addresses must be added to a carrier’s delivery route. Budget changes at local post offices can also impact delivery definitions because of staff or technology modifications. Most of these changes are minor – adding or deleting a few streets or moving a few addresses from one carrier route to another. However, sometimes an entire delivery system for a county can be disrupted as entire ZIP Codes are added or subdivided.

While the change can be initiated by local post offices, the USPS headquarters in Memphis, TN is the only entity to aggregate individual carrier route change information nationally. They release a monthly database of all carrier route definitions. There are many independent companies that license various subsets of this database to help other businesses ensure that large mailings are addressed efficiently for delivery.

Somewhere in the US, a carrier route changes every day. The USPS releases a monthly database of the latest definitions, but it does not “track” the changes per se. Some changes are very subtle – one new address added because a new home was built at the end of a street – and so tracking the type of change is an expensive effort few commercial organizations have undertaken.

Each carrier route is explicitly defined as part of a single ZIP Code. The full 9 digits of the carrier route code (e.g. 12508C007) is a unique code that represents each carrier route and so, by definition, it cannot cross multiple ZIP Codes.

Nationwide, each month you can expect at least 100,000 changes (adds, deletes, or modifications) to the carrier route definitions. That represents more than 15% each month!

Although a ZIP+4 code also has 9 digits, it is not the same as a carrier route code. There are far more ZIP+4 codes in the US – approximately 40,000,000, usually assigned to one side of a street block. Each carrier route contains tens or hundreds of ZIP+4 codes. Additional info on ZIP+4 maps and lists can be found on the web.

Carrier routes can be sub-characterized as city, rural, PO Box, or highway contract routes - each based on what kind of delivery mechanism is used. The letter at the start of the carrier route number indicates the sub-characterization. The designation "R" denotes a rural delivery carrier route where mail is delivered via automobile. A "C" designation means that it is a city delivery route, where mail is delivered on foot by a postal service employee. The designation "B" indicates that is "PO box delivery". These carrier routes cannot be mapped by mapping firms because multiple labels overlap significantly. Regardless of the sub-characterization, all postal discounts allowed for mailing by carrier route will apply. While these letter-based designations can give you some idea of the density of occupants to expect in any given carrier route, they do not limit your ability to send direct mail.

The USPS does not usually define carrier routes by the type of occupant receiving mail: any given carrier route could include any or all of the above types of recipients. However, it is possible to get summary counts for any carrier route from your list provider or mapping service bureau showing the number of deliverable addresses for businesses, MFDUs (multi-family dwelling units), and SFDUs (single family dwelling units). You can use this data to target your mailings to carrier routes heavy in the type of recipient of interest to you.

Canada’s postal system is designed a little differently than that in the US. Canada uses a 6-digit code, called the “FSALDU”, where the FSA is similar to a US ZIP Code and the LDU is more akin to a US ZIP+4.

A saturation mailing is simply any mailing campaign that mails to at least 90% of the residential addresses within a postal carrier route. Many mailers erroneously think that a saturation mailing must be conducted by ZIP Code but the actual definitions from the USPS for the maximum saturation discount uses the carrier route as the defining region.

When you mail by saturating a carrier route, you can expect to save between 50-60% on your mailing postage costs. Similar savings can be obtained by doing a “high density” mailing where you mail to at least 125 addresses in a carrier route. For the most current rates, you should work with your lettershop or list provider, or call you local post office. For current postal rates, click here.

When you prepare your mailing according to USPS guidelines, including meeting the saturation criteria, it makes the post office’s job easier. It reduces the time-consuming coding and sorting that is part of the USPS's mail delivery process.

Any business attempting to reach the majority of residents in a localized area should mail by carrier route.

A resident occupant list is a list of addresses of all residents in a particular area, prepared by a commercial list provider. It comes pre-sorted and saturated by carrier route and guarantees the maximum postal discounts.

All major direct mail campaigns should be running the address files through an address standardization process known as "CASS," following strict USPS guidelines. Even non-saturation mailings should be CASS certified prior to mailing to ensure the maximum delivery. CASS checks the addresses against a USPS file of deliverable addresses, corrects any typos in your address file, appends the ZIP+4 and carrier route codes (along with some other codes not relevant to this discussion). Your list provider should always run the list against a CASS process before giving it to you, but your lettershop or other service bureau can CASS your existing customer database for you. You can also use carrier routes to target new customers.

Because your file should always be CASS certified (see previous question), you will always have both a ZIP+4 code and a carrier route code on your address file. However, the maximum postal discount only applies when mailing to at least 90% of the residential addresses within a carrier route. There are comparatively minimal discounts for mailing by ZIP+4. Hence “mailing by carrier route” is usually most advisable.

For service bureaus that analyze your files, it is also a lot easier to work with the smaller carrier route database (~600,000 records) than the large ZIP+4 database (~40,000,000) so data processing costs are generally much lower for files sorted by carrier route.

Many demographic data providers are able to generate statistics on income, age, and other demographics based on carrier route.