ISBN Validator

How it is calculated

Spaces and hyphens are removed and letters are made upper-case. Ten characters (nine digits and a final digit or X) are checked as an ISBN-10, and thirteen digits as an ISBN-13; anything else gives no result. For ISBN-10 the first nine digits are multiplied by 10, 9, … 2, summed to S, and the check character is (11 − S mod 11) mod 11, written X when it is 10. For ISBN-13 the first twelve digits are weighted 1, 3, 1, 3 …, summed to S, and the check digit is (10 − S mod 10) mod 10.

The tool shows Valid or Invalid, the type, and the check digit the algorithm expects, which you can compare with the last character you typed. It checks the arithmetic only: it does not require the 978 or 979 prefix on a 13-digit number, so any 13 digits with a matching check digit pass, and it does not look the book up.

Common mistakes

• Typing the ISBN-10 for the same book and expecting the same check digit as the ISBN-13: the two numbers use different formulas, so the last character differs (2 and 7 for this book).
• Using a lower-case x or a digit 10 for the final character: x is accepted as X, but 10 as a two-digit number makes the code 11 characters long and gives no result.
• Entering 12 digits, or an ISSN or EAN of another length: only 10 or 13 characters are checked.
• Reading Valid as proof that the ISBN is assigned to a real book: it shows only that the check digit fits, and any 13-digit EAN with a correct check digit also passes.