PostNet (3 of 5)
Home > Barcode Basics > Different Types of Barcodes > PostNet

The PostNet barcode is used by the United States Postal Service to automatically sort mail. The PostNet code consists of evenly spaced bars of two different heights. Each character is represented by five bars, two tall and three short. The character set includes the digits 0 through 9. The code begins and ends with a tall bar ('frame bar'), and may contain a 5-digit ZIP code, a 9-digit ZIP+4 code, or an 11-digit Delivery Point Code. A Modulo 10 check digit ('correction character') is inserted after the ZIP code and before the ending frame bar.

The check digit is calculated on the content of each ZIP code. The check digit is simply the number that, when added to the sum of the digits in the ZIP code, will produce an even multiple of 10.

Here is some sample 'C' code that will calculate the checksum.  This function will work with any length of zip code (it will not generate an error if the zip code is an incorrect length), and it will automatically skip over any embedded dashes or other non-numeric characters.  Note that dashes have to be removed before printing the barcode:

int checksum(char *zipcode)
{
  int tot;
  char *cp;

  tot = 0;
  cp = zipcode;
  while (*cp != 0) {
     if (*cp >= '0' && *cp <= '9')
        tot += *cp - '0';
     cp++;
     }
  return(10 - (tot % 10));
}

Here is a routine for FileMaker Pro.  This function operates on a 9-digit zip code and assumes that there are no embedded dashes.  CALCTOT adds up the digits and CHECKSUM calculates the check digit:

ZIP Text

CALCTOT Calculation(Number) = TextToNum(Left(ZIP,1)) + TextToNum(Middle(ZIP,2,1)) + TextToNum(Middle(ZIP,3,1))  + TextToNum(Middle(ZIP,4,1)) + TextToNum(Middle(ZIP,5,1)) + TextToNum(Middle(ZIP,6,1)) + TextToNum(Middle(ZIP,7,1)) + TextToNum(Middle(ZIP,8,1)) + TextToNum(Middle(ZIP,9,1))

CHECKSUM Calculation(Number) = 10 - MOD(CALCTOT,10)

 

See also:
AppNote 003:   Calculating the PostNet Barcode Checksum in Access Basic