ISBN (International Standard Book Number) is a unique numeric identifier for books, e-books, and other published media. It enables publishers, libraries, and retailers to track and manage books accurately across the global supply chain. Every distinct format of a book (hardcover, paperback, EPUB, etc.) receives its own ISBN.

The system was introduced in 1970 with 10-digit codes (ISBN‑10). In 2007, the standard expanded to 13 digits (ISBN‑13) to align with the global EAN barcode system and accommodate growing publication volumes.

• ISBN‑10: 10 digits (0–9, with 'X' allowed as the final check digit representing 10). Uses modulo‑11 checksum with weights 10 down to 1.
• ISBN‑13: 13 digits (0–9 only). Uses modulo‑10 checksum with alternating weights of 1 and 3. Prefixed by 978 or 979 (the "Bookland" EAN prefix).
• An ISBN‑13 starting with 978 can usually be converted back to its ISBN‑10 equivalent (and vice versa).

ISBN‑10: Multiply each of the first 9 digits by weights 10, 9, 8, …, 2. Sum the results. The check digit is the value that makes the total divisible by 11. If the required value is 10, the check digit is 'X'.

ISBN‑13: Multiply each of the first 12 digits by alternating weights 1, 3, 1, 3, …, 1, 3. Sum the results. The check digit is (10 − (sum mod 10)) mod 10.

Example (ISBN‑13 978-0-306-40615-7): sum = 1×9 + 3×7 + 1×8 + 3×0 + 1×3 + 3×0 + 1×6 + 3×4 + 1×0 + 3×6 + 1×1 + 3×5 = 9+21+8+0+3+0+6+12+0+18+1+15 = 93; 93 mod 10 = 3; check digit = 10−3 = 7 ✓

Yes! To convert a valid ISBN‑10 to ISBN‑13:

1. Take the first 9 digits of the ISBN‑10 (drop the original check digit).
2. Prefix them with 978.
3. Recalculate the check digit using the ISBN‑13 modulo‑10 formula.

Reverse conversion (ISBN‑13 → ISBN‑10) is only possible when the ISBN‑13 starts with 978. Drop the 978 prefix and the last digit, then recalculate the ISBN‑10 check digit (which may be 'X').

An ISBN is divided into 5 segments (shown with hyphens or spaces in printed form):

• GS1 Prefix (ISBN‑13 only): Always 978 or 979 — identifies the product as a book.
• Registration Group: Identifies the country, language, or geographic region (e.g., 0 or 1 for English-speaking countries, 2 for French, 3 for German, 4 for Japan, etc.).
• Registrant (Publisher): Identifies the specific publisher within the group.
• Publication (Title): Identifies the specific edition and format of the book.
• Check Digit: A single digit (or 'X' for ISBN‑10) used to validate the entire number.

Almost. The barcode printed on most books is the EAN‑13 barcode representation of the ISBN‑13. For ISBN‑10 books published before 2007, the barcode often contains the ISBN‑10 encoded in EAN‑13 format with a 978 prefix. The human-readable number above the barcode is the ISBN itself. The barcode also typically includes a 5-digit add-on code for the retail price.

Common reasons an ISBN fails validation:

• Transcription error: A digit was mistyped or swapped with another (e.g., typing "40651" instead of "40615").
• Wrong check digit: The check digit was miscalculated or incorrectly transcribed. Our tool shows the correct check digit.
• Wrong length: Not 10 or 13 digits after removing hyphens and spaces.
• Invalid characters: ISBN‑13 must be all digits; ISBN‑10 allows 'X' only as the final character.
• Fake or placeholder ISBN: Some websites use placeholder ISBNs that aren't mathematically valid.

Absolutely. Our validator automatically strips hyphens, spaces, and any "ISBN" prefix before validation. You can paste ISBNs in any common format: 978-0-306-40615-7, 0 306 40615 2, ISBN: 9780306406157, or just the raw digits. The cleaned version is displayed alongside the results so you can verify the parsing.

The 978 and 979 prefixes are part of the GS1 (Global Standards One) system. The International ISBN Agency secured 978 in the 1980s as the "Bookland" prefix — a reserved EAN identifier that tells barcode scanners worldwide: "this is a book." When the 978 range began running out of capacity, the 979 prefix was activated in 2020. Books with 979 ISBN‑13 do not have an ISBN‑10 equivalent because the ISBN‑10 system was retired in 2007.