Processed locally
Your input stays in this browser.
One checksum rule covers several lengths
GTIN-8, UPC-A/GTIN-12, EAN-13/GTIN-13 and GTIN-14 all use the same modulo-10 check digit. The length changes the business representation; it does not change the alternating 3-and-1 weighting. Enter a body to calculate its final digit, or enter the complete number to test it. Spaces and hyphens are ignored, but letters and an unsupported length are rejected rather than silently removed.
Work the number from the right
Take the body 950110153000. From its right edge, multiply digits by 3, 1, 3, 1 and so on: 0×3 + 0 + 0×3 + 3 + 5×3 + 0 + 1×3 + 1 + 0×3 + 1 + 5×3 + 9 = 47. The next multiple of ten is 50. The difference is 3, so the full GTIN-13 is 9501101530003.
The compact formula
Let S be the weighted sum of all digits except the check digit. The result is c = (10 - (S mod 10)) mod 10. The final mod matters: when S already ends in zero, c is zero rather than ten. The calculator displays the individual products as well as S, which makes a transposed digit easier to audit than a bare pass or fail message.
What a passing result proves
A valid check digit proves only that this character sequence satisfies the arithmetic rule. It does not prove that the number was assigned by GS1, that it identifies the package in front of you, or that a retailer has it in its catalogue. A checksum is intentionally cheap: it catches many single-digit and some transcription errors without turning a barcode scanner into a registry.
What it does not catch
Changing two digits can preserve the same weighted sum, and swapping digits with equal weights leaves the result unchanged. For example, positions carrying the same multiplier can be exchanged with no checksum alarm. Compare the full identifier with the master data source when correctness matters. This calculator is therefore a data-entry check, not a substitute for supplier onboarding or a product-information system.
Choose the length before printing
A 12-digit UPC-A can be represented in EAN-13 by adding a leading zero, while GTIN-14 often represents a packaging level. They can share a numeric core yet describe different barcode uses. The calculator deliberately tests digits without choosing a symbology. Use a barcode generator only after deciding whether the target is a shelf item, a compact package, or a logistics unit.
Common input mistakes
People frequently include the leading AI 01 from a GS1-128 element string, paste a price code beside the GTIN, or count a visible leading zero as decoration. The rule is simple: test the identifier exactly as its chosen representation requires. For AI 01, test the 14 digits after the AI; do not include the parentheses or the AI itself in the GTIN arithmetic.
Why this page differs from the converter
This page explains and verifies modulo-10 arithmetic for every supported GTIN length. The UPC-A to EAN-13 converter performs only the standardized leading-zero representation change and rejects non-UPC input. Neither page generates a barcode image, assigns product identifiers, or turns a valid mathematical sequence into permission to use it commercially.
Use the result in a traceable review
Record the submitted body, the weighted sum and the resulting digit when correcting imported data. If the supplied complete number fails, ask whether the body or final digit is wrong; do not assume the last character is the mistake. The displayed calculation is deterministic and entirely local to the browser, so confidential item data is not sent to this site.
Compare the same representation
When a spreadsheet stores a GTIN as a number, leading zeros can disappear before the checksum is calculated. Store identifiers as text and preserve their intended length. A result of 36000291452 is not the same representation as UPC-A 036000291452, even though the latter has one added leading zero. The calculator accepts the digits you enter; it cannot infer zeros lost by a database column.
Use a second source for allocation
For a supplier dispute, compare the checked number with the supplier's approved product master or the relevant GS1 data source. The weighted arithmetic can prove that a digit sequence is self-consistent, but it cannot decide which of two self-consistent records belongs to the actual package. That division of responsibility keeps a local calculator from becoming an accidental authority over commercial identifiers.
Keep correction auditable
If a complete value fails, preserve the original string alongside the recalculated candidate and ask the source-system owner to approve any correction. A support ticket saying only 'checksum fixed' loses the most important evidence: which input was received and why it changed. The displayed products and sum make this page useful for that narrow review without uploading catalogue data.
Keep the verification boundary clear
Use the calculation as evidence at the point of entry, then retain the original identifier and its authoritative source. That prevents an arithmetic pass from being mistaken for a commercial allocation decision or a catalogue approval. For recurring imports, reject unsupported lengths before calculation and log the exact character count; a visibly correct number in a spreadsheet can still have lost a leading zero during export.