How is an ISBN-10 check digit calculated?

Multiply the first 9 digits by weights 10 down to 2, add the check digit times 1, and the ISBN is valid if the total is divisible by 11. The check digit is the value 0–10 that makes the sum divisible by 11, where 10 is written as X.

Why do some ISBN-10s end in X?

The check digit can be any value from 0 to 10. Because 10 is not a single decimal digit, ISBN-10 writes it as the letter X. This only ever appears in the final position.

Can I compute a missing check digit?

Yes. Enter just the first 9 digits and the tool shows the correct check digit and the full 10-character ISBN.

Are hyphens required?

No. Hyphens and spaces are ignored, so you can paste an ISBN grouped as 0-306-40615-2 or as a plain 10-character string.

Is my ISBN sent to a server?

No. The checksum runs entirely in your browser, so the number never leaves your device.

ISBN-10 Validator & Check Digit

An ISBN-10 (International Standard Book Number, 10 characters) is the older book identifier defined by ISO 2108 and used for titles published before 2007. The final character is a check digit that lets you catch typos. This free validator runs the official mod-11 checksum and can also compute the correct check digit when you enter only the first nine digits.

How it works

ISBN-10 uses a weighted modulus-11 scheme:

Strip hyphens and spaces, leaving 10 characters.
Multiply the digits by descending weights: position 1 by 10, position 2 by 9, down to position 9 by 2, and the check character by 1.
The check character may be 0–9 or X, where X represents the value 10.
Add all the products together. The ISBN is valid if the total is divisible by 11.

To generate the check digit for the first nine digits, compute (11 - (weighted sum) mod 11) mod 11; a result of 10 is written as X.

Example

For 0-306-40615-2, the first nine digits weighted give 0×10 + 3×9 + 0×8 + 6×7 + 4×6 + 0×5 + 6×4 + 1×3 + 5×2 = 130. The check digit 2 contributes 2×1 = 2, for a total of 132, which is divisible by 11, so the ISBN is valid.

If validation fails, the tool shows the expected check digit so a single mistyped or transposed digit is easy to spot. All processing is local.