Check digit
A check digit is the last digit of a barcode number, worked out from the digits before it so that a typing mistake can be caught.
- Checksum
What it means
Every GTIN, the number printed under a barcode, ends in a check digit. It plays no part in identifying the product. It is calculated from all the other digits, and GS1 US describes it as the digit that “helps to confirm the integrity of your barcode number”.1
The sum is simple enough for a spreadsheet. Set the last digit aside. Starting from the right, multiply the remaining digits by 3, then 1, then 3, and so on, and add up the results. The amount needed to round that total up to a multiple of 10 is the check digit.
If any one digit is mistyped, the total changes and the number fails. A pass tells you the number is well formed. It does not tell you the number belongs to your product: a real barcode pasted onto the wrong row passes every check there is.
Think of it like this
It is like the total at the bottom of a till receipt. Add up the lines yourself, and if your sum does not match, you know something was keyed in wrong, even before you know which line.
For example
Take 501234567890, the first twelve digits of an example barcode. Multiplied by 3 and 1 in turn from the right, they add up to 90, which is already a multiple of 10, so the check digit is 0 and the full number is 5012345678900. Type a 6 where the 7 should be and the total drops to 89, so the check fails.
Why it matters
Spreadsheets and hurried typing damage barcodes all the time, and a damaged barcode can point a listing at someone else’s product. Google asks for a check digit that is “present and correct”,2 and running the check on every row finds the broken numbers before a channel does.
Not to be confused with
Sources
- GS1 US. Check Digit Calculator.
- Google Merchant Center Help. GTIN [gtin].