Code 39 Barcode Specification

Following on from last weeks post about how store barcodes work, and the limitations of checksums which is often missed, here is a guide to what makes a Code 3 of 9 Barcode.

The basic principle is that it is two different widths of bar/space (kind of like the principle of morse code) which represent symbols from a set of 43 characters containing letters, numbers and some special characters (punctuation essentially). Each of the characters is made up of 5 bars and 4 spaces with a space between each character. Therefore by simply counting the number of bars you should be able to work out whether you have a valid code 39 barcodes.

Code 39 Barcode

Code 39 Barcode

The checksum is seen by many as a guarantee that you don’t receive a misread. However if you understand how it works then it becomes clear that it only reduces the chance of a misread. Code 39’s checksum is modulus 43, which in English means that if you add up the value of all the characters (letters have numeric values too) and then subtract 43 you are left with your checksum. I have included an example below.

For example, if you are encoding the string ABCD1234 then the checksum values will be 10 + 11 + 12 + 13 + 1 + 2 + 3 + 4 = 56.

56 mod 43 = 13. So the checksum character should be D.

Therefore although greatly reducing the chance of getting a false read it can still produce a misread in a certain percentage of cases, which may be a very low percentage. However if you are processing thousands of barcodes a day, relying on checksums to make up for shoddy scanning, don’t be surprised at various anomalies.

For more information please see our website’s knowledge base. If there is anything else you want to know about Code 39 barcodes, or any other barcodes, then just comment below!


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