ISBN Validator
Validate ISBN-10 and ISBN-13 check digits.
Overview
The ISBN Validator checks whether a 10-digit or 13-digit International Standard Book Number is well-formed and has a valid check digit. Paste a string with or without hyphens — the validator strips formatting, runs the right algorithm for the length and tells you whether the code is valid plus the standard form to use in catalog records.
The tool is used by librarians cleaning a catalog, second-hand booksellers listing inventory and software developers building a book-search app. Long-tail queries it covers include "ISBN check digit calculator", "validate ISBN-13 online" and "what makes an ISBN invalid".
How it works
ISBN-10 uses a modulo-11 weighted sum: multiply the digits by 10, 9, 8 down to 1 from the left and sum the products. A valid ISBN-10 has a total that is a multiple of 11. The check digit (the last digit) can be 0-9 or X — X represents the value 10. Calculating the expected check digit means finding the value that makes the running sum a multiple of 11.
ISBN-13 uses the EAN-13 algorithm: weight odd-position digits by 1 and even-position digits by 3, sum, then take the result mod 10 and subtract from 10 (with 10 mapped back to 0). The validator strips hyphens and spaces, normalises X to uppercase, then runs the right algorithm based on the length.
Examples
0-13-110362-X → Valid ISBN-10
978-0-13-110362-7 → Valid ISBN-13
9780306406157 → Valid ISBN-13
0-13-110362-9 → Invalid (check digit should be X)
FAQ
Why does ISBN-10 use X?
The modulo-11 check digit needs an 11th symbol for the value 10. The convention is the Roman numeral X, which is a single character and unambiguous.
Can I convert ISBN-10 to ISBN-13?
Yes. Prepend 978 to the first 9 digits of the ISBN-10, then recompute the check digit using the EAN-13 algorithm. Most modern systems do this transparently.
Are hyphens part of the ISBN?
The digits are the ISBN; hyphens are decorative groupings of country, publisher and title segments. They're useful for humans but ignored by every algorithm and database.
Why is my new book's ISBN different from the old edition?
A new edition or significantly different physical format (hardcover vs paperback) gets a new ISBN. Reprints with no content change keep the same ISBN.
Does ISBN guarantee a book is unique?
No, only that the code is well-formed. Two unrelated publications can in theory share an ISBN if a publisher recycles a number, though the registration system makes this rare.