ISBN Converter & Validator
Paste an ISBN in either form — you get both, validated with the right algorithm for each. One book, one line, no format picker.
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Two forms, two completely different check digits
This is where most ISBN code goes wrong. An ISBN-10 uses a modulo-11 checksum with weights 10 down to 2 — and when the check value comes out as 10, it is written
as the letter X. An ISBN-13 uses the GS1 modulo-10 checksum, exactly like any other barcode.
They are not interchangeable. Running the barcode algorithm on a 10-digit ISBN produces a wrong check digit every time, which is why so many "validators" reject perfectly good ISBNs that end in X.
Converting between them
To go from 10 to 13: prefix 978, drop the old check character, recalculate with
the GS1 algorithm. The nine digits in the middle never change — that is the book's real
identity. Going back the other way just reverses it.
Why some ISBNs have no 10-digit form
The 979 prefix was introduced after ISBN-10 was retired in 2007. Books
issued under it have never had a 10-digit form and never will. A tool that hands you one is
generating a number that identifies nothing — so this one tells you it does not exist instead.
One sub-range is special: 979-0 is reserved for ISMN, the
identifier for printed music. It scans like an ISBN but belongs in a music-publication field.
Every check this tool runs (13 rules)
Format & length
ISBN-F01 Not a valid ISBN length
An ISBN is either 10 characters (the pre-2007 form) or 13 digits (the current form). Anything else is not an ISBN — 9 digits usually means a lost leading zero, 12 means a truncated ISBN-13.
Fix: Check the number on the copyright page. A 9-character value is often an ISBN-10 that lost its leading zero.
Source: ISO 2108 (ISBN) and the ISBN International registration-group structure. Rule wording and edge cases are verified against real published ISBNs in our regression tests.
ISBN-F02 Invalid characters
Only digits are allowed, except for the final character of an ISBN-10, which may be X (representing the check value 10). A lowercase x is accepted and normalised.
Fix: Remove letters and symbols. If the last character is x, write it as uppercase X.
Source: ISO 2108 (ISBN) and the ISBN International registration-group structure. Rule wording and edge cases are verified against real published ISBNs in our regression tests.
ISBN-F03 X used somewhere other than the check position
X may only appear as the last character of an ISBN-10, where it stands for the check value 10. Anywhere else it is not valid.
Fix: Correct the digit — X is not a digit, it is a check value.
Source: ISO 2108 (ISBN) and the ISBN International registration-group structure. Rule wording and edge cases are verified against real published ISBNs in our regression tests.
ISBN-F04 ISBN-13 without a Bookland prefix
A 13-digit ISBN must start with 978 or 979. A 13-digit number starting with anything else is a regular GTIN, not an ISBN.
Fix: If this is a product barcode rather than a book identifier, validate it as a GTIN instead.
Source: ISO 2108 (ISBN) and the ISBN International registration-group structure. Rule wording and edge cases are verified against real published ISBNs in our regression tests.
ISBN-F05 Hyphens and spaces removed
ISBNs are commonly printed with hyphens that mark the registration group, registrant and publication elements. They carry no data for validation and were stripped before checking.
Source: ISO 2108 (ISBN) and the ISBN International registration-group structure. Rule wording and edge cases are verified against real published ISBNs in our regression tests.
Check digits (two different algorithms)
ISBN-C01 ISBN-10 check digit is wrong
The last character of an ISBN-10 is a modulo-11 check value: the first nine digits are weighted 10 down to 2, summed, and the check value brings the total to a multiple of 11. A check value of 10 is written as X. This is a different algorithm from the one used by ISBN-13.
Fix: Compare the number against the copyright page. Do not simply replace the check character.
Source: ISO 2108 (ISBN) and the ISBN International registration-group structure. Rule wording and edge cases are verified against real published ISBNs in our regression tests.
ISBN-C02 ISBN-13 check digit is wrong
An ISBN-13 uses the GS1 modulo-10 checksum (alternating weights 1 and 3 from the left), the same algorithm as any other GTIN-13.
Fix: Compare the number against the barcode on the back cover.
Source: ISO 2108 (ISBN) and the ISBN International registration-group structure. Rule wording and edge cases are verified against real published ISBNs in our regression tests.
Conversion & equivalence
ISBN-V01 ISBN-10 ⇄ ISBN-13 equivalence
An ISBN-10 becomes an ISBN-13 by prefixing 978 and recalculating the check digit with the GS1 algorithm. The nine digits in between never change — the two forms identify the same book.
Source: ISO 2108 (ISBN) and the ISBN International registration-group structure. Rule wording and edge cases are verified against real published ISBNs in our regression tests.
ISBN-V02 No ISBN-10 form exists (979 prefix)
Only ISBNs beginning 978 have a 10-character form. The 979 prefix was introduced after ISBN-10 was retired, so books issued under it have never had one — this is not an error, and any tool that "converts" a 979 ISBN to 10 characters is inventing a number.
Source: ISO 2108 (ISBN) and the ISBN International registration-group structure. Rule wording and edge cases are verified against real published ISBNs in our regression tests.
ISBN-V03 979-0 is an ISMN, not an ISBN
The 979-0 range is reserved for ISMN — the identifier for printed music (sheet music, scores). It is a valid barcode but belongs in a music-publication field, not an ISBN field.
Source: ISO 2108 (ISBN) and the ISBN International registration-group structure. Rule wording and edge cases are verified against real published ISBNs in our regression tests.
Registration groups
ISBN-G01 Registration group
The element after the 978/979 prefix identifies the registration group — a national, geographic or language area. It tells you which ISBN agency issued the number, not where the book was printed or what language it is in.
Source: ISO 2108 (ISBN) and the ISBN International registration-group structure. Rule wording and edge cases are verified against real published ISBNs in our regression tests.
ISBN-G02 Registration group not in our table
The registration group is not in this tool's table. ISBN International adds groups over time, so this alone does not make the ISBN invalid.
Source: ISO 2108 (ISBN) and the ISBN International registration-group structure. Rule wording and edge cases are verified against real published ISBNs in our regression tests.
Plausibility
ISBN-P02 All identical or sequential digits
Values like 0000000000 or 1234567890 are placeholders. Some of them satisfy the checksum by coincidence, which is exactly why a checksum-only validator is not enough.
Fix: Use the ISBN printed on the book.
Source: ISO 2108 (ISBN) and the ISBN International registration-group structure. Rule wording and edge cases are verified against real published ISBNs in our regression tests.
Not yet supported: hyphenation (splitting an ISBN into prefix, registration group, registrant and publication elements). Doing that correctly requires the official ISBN registrant range data, which we have not integrated — and guessing the split would produce confidently wrong output. Group identification is shown instead.