Have you ever wondered about the barcode at the bottom of an envelope? Humans don’t typically have to read the barcodes printed on envelopes by the United States Postal Service. That is why we have machines. Nonetheless, a postal barcode can be read with the naked eye if you know the right value for each number. In fact, if a machine makes an error, a human sometimes does have to step in and correct it.

As a business owner who is likely not delivering the mail (unless you happen to moonlight as a mail carrier), the postal barcode can actually tell you a whole lot about an address anyway. The series of numbers within it are all based on ZIP codes and location.

What Is the Barcode at the Bottom of the Envelope?

Any item going through the U.S. mail will be assigned what is called a POSTNET barcode, and you can find this printed somewhere on your envelope. POSTNET stands for "postal numerical encoding technique." The code is a series of long and tall bars. These bars represent certain numerals. As long as you know which numerals correspond to which series of bars, you can reasonably read the actual barcode yourself.

How to Read a POSTNET Barcode

Every POSTNET barcode includes at least 32 lines. The first and last long lines mark the beginning and end of the code so it can easily be read by machines. The remaining lines are split into six groups of five bars. Each one of these five bars represents a single digit. The long bars are indicated as “I,” and the short bars are indicated as “.”

  • 1: ...II
  • 2: ..I.I
  • 3: ..II.
  • 4: .I..I
  • 5: .I.I.
  • 6: .II..
  • 7: I...I
  • 8: I..I.
  • 9: I.I..
  • 0: II...

So, a simple five-digit POSTNET code showing I.I.I..II...I..I.I.I.I..I...I.II would be read as 564582.

Where Do the Numbers in a POSTNET Barcode Come From?

If you are wondering about the barcode at the bottom of the envelope, there is a hyper-specific rhyme and reason as to why certain numbers appear. There are three different types: five-digit POSTNET barcodes, ZIP+four-digit POSTNET barcodes and delivery-point barcodes (or DPBC). Each one is comprised of a certain string of numbers, as follows:

  • The five-digit POSTNET barcode: This includes the start and stop characters, the five-digit ZIP code and an additional check digit. It’s a total of 32 bars.

  • The ZIP+four POSTNET barcode: This includes the start and stop characters, a nine-digit ZIP code and an additional check digit. It’s a total of 52 bars.

  • The DPBC barcode: This includes the start and stop characters, a nine-digit ZIP code, two delivery-point numbers and a check digit. To get the delivery-point numbers, you’ll usually use the last two digits of a street address, PO box or route number. This code is 62 bars.

What’s That Random Last Number?

The last number on a POSTNET barcode is considered the check digit or checksum. This number is determined by adding up the sum of all the numbers in a barcode and figuring out what single number would create a multiple of 10.

For example, if you have a barcode that reads 33727-1426, you would add 5 as the checksum because 3+3+7+2+7+1+4+2+6 = 35, and 35+5 = 40. 40 is the closest multiple of 10 you could get.