Bulk GTIN validation modes

The bulk validator on the check digit calculator returns a verdict for every line of a pasted column. This guide explains how that engine reads a line: the order of operations, the two reading modes, the four statuses, the corrected column and the row cap. Every figure below is computed at build time by the same code that runs in your browser, on a fictional list and on the published vectors the engine is tested against; nothing is typed in.

The short version: a column that already carries its check digits stays in auto mode; a column still waiting for its check digits needs data-only mode. In the tool the switch reads “Numbers include their check digit (validate)” and “Data only — append the check digit”; this page uses the engine's names, auto and data-only. The rest is why the distinction exists and what happens when the mode does not match the column.

What happens to every line, in order

The engine processes pasted text in a fixed order:

1. Split and trim. Lines are split at any line ending and trimmed; blank lines are skipped and never counted.
2. Strip separators. Spaces and hyphens are removed; the line as pasted stays the row's input, the digits its cleaned form.
3. Enforce digits only. A separator-only line reports “No digits on this line.”; a line with other characters reports “Contains characters other than digits, spaces, and hyphens.”
4. Read the digit count by mode. Auto verifies 8, 12, 13 and 14 digits as complete numbers and completes 7 and 11; data-only completes 7, 11, 12 and 13. Any other count is not usable.
5. Verify or compute, never repair. A verified line keeps its digits and its status says whether the final digit matched; a completed line gets the computed digit appended and is labeled completed, not valid.
6. Report. Each row carries its family, status, expected check digit, complete number when one exists and a one-sentence note; the report adds four counts, the non-blank total and whether the cap was hit.

The four lengths are the GTIN families this engine covers — the numbers inside EAN-8, UPC-A, EAN-13 and ITF-14 symbols — sharing one mod-10 computation, walked through in how GTIN check digits are computed.

Auto mode: the complete-number reading wins

A 12-digit line could be a complete GTIN-12 or the first twelve digits of a GTIN-13 awaiting its check digit; a 13-digit line is ambiguous in the same way between a complete GTIN-13 and GTIN-14 data. The engine resolves this openly: in auto mode the complete-number reading wins, because the tool is a validator first. Seven- and eleven-digit lines have no complete reading, so they are the only lines auto mode completes.

The fictional list below shows both readings side by side. Rows 1 to 3 come from the fictional data digits 730112345678 and the engine's own check digit: the complete number, the same number with its last digit moved by one, and the complete number pasted with a space and a hyphen inside it. Rows 4 and 5 are seven and eleven fictional data digits. Rows 6 and 7 are twelve and thirteen data digits with no check digit — the trap. After a skipped blank line come a five-digit fragment and a value pasted in scientific notation.

Row 3 cleaned to the same digits as row 1, so both verify. Row 2's note reads “Check digit should be 5, not 6.” — the never-repair rule in one sentence: the expected digit is named, the complete number sits in its own column, and the input is left as pasted. Rows 6 and 7 are the trap: each was read as a complete number and its last data digit tested as a check digit it never was. Both verdicts answer the question auto mode asks — is this a complete number with a fitting final digit? — not the question the person pasting meant. The calculator's single-number form names that road not taken; for a whole column, the alternative is the second mode.

Data-only mode: every line is data digits

Switch the mode and the same text reads differently, as the last column shows: 7, 11, 12 and 13 digits become GTIN-8, GTIN-12, GTIN-13 and GTIN-14 data, each gets a computed check digit appended, and every successful row is labeled completed — nothing is verified, because the mode has said there is no check digit to verify.

Row 6 now completes to the number in row 1, so the trap is resolved. But row 2, the corrupted number, also succeeds: read as thirteen data digits, its wrong final digit is treated as data and a fourteenth digit appended, giving a structurally valid GTIN-14 nobody wanted. Data-only mode cannot catch a wrong check digit because it never looks for one, which is why the mode must match the column. Rows 8 and 9 stay not usable, and the note now names the lengths this mode accepts: “5 digits — data digits without a check digit come as 7, 11, 12, or 13.”

The four statuses and the corrected column

The tool shows the engine's four statuses as “Check digit correct” (valid), “Wrong check digit” (wrong-check-digit), “Check digit appended” (completed), and “Not usable” (invalid). Only the first says anything was verified; completed means a digit was computed, not checked; not usable means no number was invented.

The corrected column is where never-repair becomes visible: the engine reports the expected digit and puts the data digits plus that digit in a separate field, shown beside the input. A mismatch often means one of the other digits was mistyped, so the decision stays with whoever owns the data. Copying the column preserves row order; unusable rows become blank lines so it pastes back beside the original.

The published vectors under both readings

Both modes on the published reference values the methodology page cites and the engine's tests use:

The twelve walkthrough digits show the ambiguity most sharply. Auto mode reads them as a complete GTIN-12, finds that the final digit does not fit the eleven before it, and offers the data digits with the expected digit; data-only mode reads them as GTIN-13 data and appends the check digit, giving the EAN-13 the walkthrough is about. The eight- and fourteen-digit vectors verify in auto mode and are not usable in data-only mode — “8 digits — data digits without a check digit come as 7, 11, 12, or 13.” — while the eleven UPC-A data digits come out identical in both, as every 7- and 11-digit line does. The ISBN-13 verifies as a GTIN-13 once its hyphens are stripped, because its final digit is an EAN-13 check digit; data-only mode extends it to fourteen digits without complaint. The ISBN-10 forms are not usable in either mode: ten characters is not a GTIN length and X is not a digit. Their mod-11 check character belongs to the ISBN barcode tool and the ISBN conversion guide.

The row cap

The engine processes at most 500 non-blank lines per pass and says so when the input was longer, carrying the non-blank total and a truncation flag in its report. Fed 503 copies of the fictional complete number, it returns 500 rows, sets the flag, and the tool prints:

Blank lines do not count: the fictional list above had 10 physical lines and 9 non-blank ones. Nothing beyond the cap is validated.

Where the validator stops

• Structure, not registration. A valid status means the digits are internally consistent under the mod-10 rule. Validating a number does not allocate or license the GTIN inside it — that is GS1's role; see Do I need a GS1 barcode?
• Mod-10 only. ISBN-10 check characters, and anything that is not GTIN-8, -12, -13 or -14 data, fall outside this engine, which says so rather than guessing.
• No bars. The validator renders nothing. The bulk label tool runs each row through the same structural and check-digit validation before rendering; as the methodology page states, bulk PDF sheets are the deliberate exception whose assembled PDF is not individually decoded.

Bulk validation FAQ

Math, not optics: this validator checks digits and their mod-10 consistency. Whether a printed symbol made from them scans depends on contrast, size, substrate and preserved quiet zones, which no browser check can certify — test a printed sample with your own scanner before production use. Everything runs locally in your browser. See the methodology page for how the verification layer is tested, and the guides index for the rest of this series.