Canadian · Validation
ISBN check-digit validator.
Verify an ISBN-10 or ISBN-13's real check digit and convert between the two formats. Everything runs in your browser: nothing you type is transmitted anywhere.
Inputs
Result
Enter an ISBN and press Validate.
How the ISBN check digit works
ISBN-10 uses its own mod-11 weighted checksum. Each of the first 9 digits is multiplied by a descending weight (10, 9, 8 … 2), the 10th (check) character carries weight 1, and the total must be divisible by 11. Because a mod-11 remainder can be 10, the check character can be the letter X — but only in that 10th position, never earlier.
ISBN-13 (in force since 1 January 2007) dropped that scheme in favour of the same mod-10 weighted algorithm GS1 uses for EAN-13/UPC retail barcodes: weights alternate 1, 3, 1, 3 … across the first 12 digits, and the check digit is whatever makes the total a multiple of 10. Every ISBN-13 is therefore also a scannable EAN-13 barcode.
Converting between the two
ISO 2108 defines the conversion: take an ISBN-10, drop its own check digit, prepend the "978" Bookland prefix to the remaining 9 digits, and recompute a fresh check digit with the EAN-13 algorithm. A 979-prefixed ISBN-13 has no ISBN-10 equivalent — that prefix range was opened only after the 10-digit numbering space ran out.
What a passing checksum does and doesn’t tell you
A valid check digit only proves the characters are internally consistent — it does not confirm the ISBN was ever actually assigned to a real published book, or tell you what that book is. For that, look the number up in a library catalogue or a service like isbnsearch.org. This tool runs entirely in your browser: nothing you type is transmitted, stored, or logged.