What does the 'X' in ISBN-10 mean?

ISBN-10 uses mod 11, so the check 'digit' can be 0–10. To keep the format 10 chars, 10 is written as 'X' (roman numeral). ISBN-13 uses mod 10 so the check is always 0–9, no X needed.

Can every ISBN-13 convert to ISBN-10?

Only if it starts with 978. ISBNs starting with 979 (introduced in 2013 as 978 ran low) have no valid ISBN-10 — older databases that expect ISBN-10 will simply not match those books.

Why are there so many hyphenations of the same ISBN?

The hyphens mark prefix / registration group / publisher / title segments, and the segment lengths vary by country and publisher. Validation ignores them. Any hyphenation of the same digits is the same ISBN.

Is ISBN the same as the barcode on the back of a book?

Yes, for modern books — the ISBN-13 IS the EAN/GTIN-13 barcode. The 5-digit add-on you sometimes see beside it encodes the price.

ISBN Validator (ISBN-10 / ISBN-13)

An ISBN (International Standard Book Number) uniquely identifies a book edition. ISBN-10 (used until 2007) has a mod-11 checksum that allows 'X' as digit 10. ISBN-13 (used since 2007, also a GTIN-13 barcode) has a mod-10 checksum like UPC codes. This tool detects the format from length, validates the checksum, and offers automatic conversion: ISBN-10 → ISBN-13 by prepending '978' and recomputing the check; ISBN-13 ↔ ISBN-10 only when the prefix is 978 (979-prefixed ISBNs have no ISBN-10 equivalent).

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ISBN

Enter an ISBN above (10 or 13 digits).

Validation = checksum only. It tells you the digits are well-formed, not that a book with that ISBN actually exists. Use a library catalog for that.

How to use

Paste an ISBN (hyphens, spaces, lowercase 'x' all fine — they're normalized).
Read the validity status. ✓ valid means the checksum passes.
Copy the converted alternate format from the panel below.

Frequently asked questions

What does the 'X' in ISBN-10 mean?: ISBN-10 uses mod 11, so the check 'digit' can be 0–10. To keep the format 10 chars, 10 is written as 'X' (roman numeral). ISBN-13 uses mod 10 so the check is always 0–9, no X needed.
Can every ISBN-13 convert to ISBN-10?: Only if it starts with 978. ISBNs starting with 979 (introduced in 2013 as 978 ran low) have no valid ISBN-10 — older databases that expect ISBN-10 will simply not match those books.
Why are there so many hyphenations of the same ISBN?: The hyphens mark prefix / registration group / publisher / title segments, and the segment lengths vary by country and publisher. Validation ignores them. Any hyphenation of the same digits is the same ISBN.
Is ISBN the same as the barcode on the back of a book?: Yes, for modern books — the ISBN-13 IS the EAN/GTIN-13 barcode. The 5-digit add-on you sometimes see beside it encodes the price.

Guides on this topic

How Barcode Check Digits Work (EAN-13, UPC-A, EAN-8) A clear, practical explanation of the GS1 modulo-10 check digit on retail barcodes — how it is calculated, why it catches errors, and how to validate a code by hand.

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