Check Digit Calculator: GTIN, UPC, EAN and ITF-14

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What is a check digit, and how is it calculated?

The check digit is the last digit of a GTIN and it is not part of the product number. It is arithmetic added on the end so that a scanner, a spreadsheet or a feed processor can catch a mistyped or misread barcode instead of quietly selling the wrong item. GS1 calls it a modulo 10 calculation, and it works like this:

1. Write out the code without its check digit.
2. Starting from the right hand digit of that body, multiply alternate digits by 3 and by 1, beginning with 3.
3. Add all the results together.
4. Round the total up to the next multiple of ten.
5. Subtract the total from that multiple. What is left is the check digit.

Using the worked example GS1 publishes: the body 629104150021 gives a weighted sum of 57. The next multiple of ten is 60. 60 minus 57 is 3, so the full GTIN-13 is 6291041500213. Tab one shows this line by line for whatever number you enter, rather than for a fixed example.

Why does one wrong digit always fail the check?

Because the alternating weights mean any single wrong digit changes the total by an amount that cannot bring it back to a multiple of ten. Swapping two adjacent digits usually fails too, since they carry different weights. That is the whole point: it catches the two mistakes humans actually make when typing barcodes.

Does every GS1 number have a check digit?

Every fixed length numeric GS1 key does: GTIN-8, GTIN-12, GTIN-13, GTIN-14, GLN, SSCC-18 and GSIN-17 all use the same modulo 10 method. A few keys do not carry one, including CPID and GIAI, and the Global Model Number uses check characters rather than a check digit.

What is the difference between a GTIN, a UPC and an EAN?

GTIN is the umbrella term. UPC and EAN are the older regional names for two of its lengths.

They are all the same number in different clothes. A 12 digit UPC becomes a GTIN-13 by adding a leading zero, and a GTIN-14 by adding two. The check digit never changes, because a leading zero contributes nothing to the weighted sum. Google Merchant Center accepts 8, 12, 13 and 14 digit values and ignores spaces and dashes.

What is the indicator digit on a GTIN-14?

The first digit. Zero means the same item as the retail unit, just zero padded. One to eight identify packaging levels, so an inner carton and an outer case each get their own GTIN-14 with a different indicator. Nine is reserved for variable measure trade items. You cannot simply strip the first digit off a GTIN-14 to get the retail EAN unless that digit is a zero.

Does the first part of a barcode show the country of origin?

No, and this is the most widely repeated barcode myth on the internet. The first three digits are the GS1 prefix, and they identify the GS1 member organisation the brand owner licensed their company prefix from. A company registered with GS1 UK can manufacture everywhere, and frequently does. Prefixes 300 to 379 are administered by GS1 France, 060 to 139 by GS1 US, and a 12 digit GTIN has an implied leading zero, so 614141234561 has the GS1 prefix 061. Tab one resolves the prefix for you and says which organisation issued it, with that caveat attached.

How do I fix GTIN errors in Google Merchant Center?

Google reports five different GTIN problems and the wording tells you exactly what went wrong. This is the decoder.

Two practical notes. Changes take roughly 24 to 72 hours to show in Merchant Center, and product rating re-matching can take 7 to 10 days, so do not keep re-editing while you wait. And Merchant Center will only show the error, not the reason: run the feed column through tab two and you get the reason for every row in one pass.

Why does my barcode show in Shopify but not in Merchant Center?

Because storing a barcode on the variant and transmitting it to Google are two different things. This has been a long running complaint: merchants fill the barcode field, see it in the product admin, and still get missing GTIN warnings across thousands of SKUs. Work through these in order.

1. Open the product in Merchant Center and look at the final attributes, which is what Google actually received, not what your store holds.
2. Check that custom products is set to false under the Google sales channel, since marking items as custom tells Google there is no identifier.
3. Check identifier_exists. If it is true, then brand plus GTIN or MPN all have to be populated.
4. If the barcodes still are not transmitting, use a feed app or a supplemental feed to send the gtin column directly rather than relying on the default channel integration.

What is identifier_exists and when should I set it to false?

It tells Google whether the product has manufacturer identifiers at all. Set it to false for genuinely custom or handmade goods, vintage items, your own private label products that were never assigned a GTIN, and anything with no brand. Do not set it to false just to silence a warning on a branded product that does have a barcode, because that removes the product from comparisons where the identifier is what matches it.

Can I use one GTIN across variants or for a multipack?

No. Every sellable variant needs its own GTIN, and a two pack is a different trade item from a single. Reusing one number across sizes and colours is one of the most common reasons a feed gets flagged, and it is the failure that bulk validation catches fastest. Tab two flags duplicates across your whole pasted list and tells you which row the number first appeared on.

My GTIN passes the check digit test. Why is it still rejected?

A valid check digit proves the arithmetic, nothing more. Four things can still be wrong with a mathematically perfect number.

1. It is not registered. Anyone can generate a valid GTIN in ten seconds. Registration is a separate thing, and you can look a number up in Verified by GS1 or GEPIR to see whether it resolves to a real brand.
2. It belongs to somebody else. Barcodes bought cheaply from resellers were originally licensed to a company that no longer exists, so the GS1 record shows the wrong brand. Amazon rejects these outright and Google can flag a brand mismatch.
3. It is in a restricted or coupon range. Prefixes 2, 02 and 04 are restricted, and 05, 98 and 99 are coupon ranges. Both pass the checksum and both get rejected.
4. It is already on another product in your own feed. Valid, registered, and still wrong for this item.

How do I check whether a barcode is registered?

Look it up in Verified by GS1, which is the global registry of licensed numbers, or in GEPIR, which returns the licensee for a given prefix. Neither is a lookup of the specific product, but both tell you whether the number belongs to the company you think it does.

Why do my UPCs keep losing a digit?

A number formatted spreadsheet column. Excel and Google Sheets strip leading zeros and, above fifteen digits, switch to scientific notation. A 12 digit UPC starting with a zero becomes 11 digits, which is not a valid GTIN length and which Merchant Center rejects as an unsupported format. Format the column as text before you export, or re-pad it in tab three. If you see a lot of 11 digit values in a feed, this is almost always the cause rather than genuinely bad data.

How do I convert ISBN-10 to ISBN-13?

Google Merchant Center accepts ISBN-13 but not ISBN-10, so books listed with the older ten character number have to be converted. It is not a reformat, it is a recalculation.

Take the first 9 digits of the ISBN-10, drop its check character
Prefix them with 978
Calculate a fresh modulo 10 check digit over those 12 digits

The X you sometimes see at the end of an ISBN-10 stands for ten, because ISBN-10 used a modulo 11 scheme that needed an eleventh symbol. ISBN-13 uses the standard GTIN modulo 10 method, so that character never carries over. Tab three does the conversion and shows the arithmetic.

GTIN check digit calculator FAQs

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