Tool Overview

This tool checks if a book number is a valid ISBN. You enter an ISBN-10 or ISBN-13 and the tool says if it is valid or not and why.

ISBNs are used to identify books. A wrong digit or typo can point to the wrong book. Checking by hand is slow and easy to get wrong. This tool runs the official check-digit rules so you can confirm an ISBN before using it.

It is for librarians, booksellers, developers, and anyone who works with book IDs. You can use it with no special skills; just type or paste the number.

Background and Concepts

An ISBN (International Standard Book Number) is a unique code for a book. ISBN-10 has 10 characters; ISBN-13 has 13. The last character is a check digit. It is computed from the rest of the digits so that a simple formula adds up to a fixed value. If one digit is wrong or two are swapped, the check usually fails.

ISBN-10 uses a modulo-11 check: each of the first nine digits is multiplied by a weight (10 down to 2), the sum is taken, and the check digit is chosen so that the total is divisible by 11. The check digit can be 0–9 or X (for 10). ISBN-13 uses a modulo-10 check similar to many barcodes: digits in odd positions are multiplied by 1, even positions by 3, and the check digit makes the sum divisible by 10.

Doing these checks by hand is tedious. This tool strips spaces and hyphens, keeps only digits and X, then runs the right formula for 10 or 13 characters and tells you if the check digit is correct. It does not convert between ISBN-10 and ISBN-13; it only validates the format you enter.

Key Features

• ISBN-10 and ISBN-13. The tool accepts both formats. If the cleaned input has 10 characters it is treated as ISBN-10; if 13, as ISBN-13. Any other length is reported as invalid. So you can validate either format without choosing it yourself.
• Hyphens and spaces. You can paste an ISBN with or without hyphens and spaces. The tool removes them and uses only digits and the letter X (for ISBN-10 check digit). So you do not need to clean the number first.
• Check digit check. For a valid length, the tool computes the correct check digit from the first 9 (ISBN-10) or 12 (ISBN-13) digits and compares it to the last character. It shows expected and found check digits so you can see why a number failed or fix the last digit.
• Clear errors and suggestions. When the number is invalid, the tool lists errors (e.g. wrong length, invalid characters, check digit mismatch) and may suggest what to check (e.g. missing digits, wrong copy). This helps you correct the number quickly.
• Explanation. A short explanation describes the result (e.g. standard format with valid check digit, or structure correct but check digit wrong). This helps you understand what passed or failed.
• Sample numbers. You can tap preset buttons to fill the input with example ISBNs and run validation. This helps you try the tool without typing a long number.
• Clear and validate. You can clear the input and enter a new number, or submit to validate. Input length is limited so the page stays responsive.
• Optional book details. When the ISBN is valid, you can request optional book information (e.g. title, author, summary). This is optional and not part of validation.

Common Use Cases

Checking a book before order or catalog. You have an ISBN from a form, spreadsheet, or barcode and want to confirm it is valid before sending an order or updating a catalog. Enter the number and use the result and check-digit details to fix typos if needed.

Forms and apps. Your app or form collects ISBNs. Use the tool to test valid and invalid examples and to see the exact error messages and expected check digit so you can show them in your app.

Data cleanup. You have a list of ISBNs with spaces or hyphens. The tool accepts both; use it to verify each number and see which ones fail and why.

Learning. You want to see how ISBN-10 and ISBN-13 check digits work. Enter a number and use the expected vs found check digit and the explanation to understand the algorithm.

Support. A user reports a wrong or rejected ISBN. Validate it in the tool, check the expected check digit, and suggest the correct last character if the error is only in the check digit.

How to Use This Tool (Step-by-Step)

1. Open the ISBN validator page.
2. Type or paste an ISBN in the input box. You can use hyphens and spaces; they will be removed. Only digits and the letter X (for ISBN-10) are used. Maximum length is 50 characters.
3. Tap Validate (or press Enter). The tool removes spaces and hyphens, then checks the length and runs the correct formula for 10 or 13 digits.
4. Read the result. Valid ISBN means the length is 10 or 13, all allowed characters are correct, and the check digit matches. Invalid ISBN means wrong length, invalid characters, or check digit mismatch. The tool shows the type (ISBN-10 or ISBN-13), the expected and found check digits, and an explanation.
5. If invalid, read the errors and suggestions. Fix the number (e.g. add missing digits, correct the last digit using the expected check digit) and validate again.
6. Optionally tap a preset (e.g. ISBN-10, ISBN-13, Example) to fill the input and validate a sample. Use clear to empty the input and enter another number.
7. If the ISBN is valid, you can optionally request book details. This is not required for validation.

Calculations and Logic

The tool uses the official ISBN check-digit formulas only.

Input is cleaned: hyphens and spaces are removed, and the string is uppercased so "x" becomes "X". Only digits and X are allowed; any other character makes the result invalid. If the cleaned string has 10 characters, it is validated as ISBN-10: the first 9 must be digits, the 10th can be a digit or X. The ISBN-10 check digit is computed as follows: sum = digit[0]*10 + digit[1]*9 + ... + digit[8]*2, remainder = sum % 11, check = (11 - remainder) % 11; if check is 10 the character is X, otherwise the digit. If the computed check equals the 10th character, the ISBN-10 is valid. If the cleaned string has 13 characters, it is validated as ISBN-13: all 13 must be digits. The ISBN-13 check digit is computed as follows: sum = digit[0]*1 + digit[1]*3 + digit[2]*1 + ... + digit[11]*3, remainder = sum % 10, check = (10 - remainder) % 10. If the computed check equals the 13th digit, the ISBN-13 is valid. Any other length (e.g. 11 or 12 digits) is invalid; the tool may suggest adding or removing digits. The result includes the expected and actual check digits so you can correct the last character if needed.

Tips, Limitations and Best Practices

Only digits and X. For ISBN-10 the last character can be X; for ISBN-13 only digits are allowed. Letters other than X or any symbols make the input invalid. The tool strips only spaces and hyphens.

Length. A valid ISBN has exactly 10 or 13 characters after cleaning. No other length is valid. If you have 11 or 12 digits, check for a missing or extra digit.

No conversion. The tool does not convert between ISBN-10 and ISBN-13. It only validates the number you enter. To get an ISBN-13 from an ISBN-10 (or the other way), use a different tool or reference.

Check digit fix. If the only error is check digit mismatch, the tool shows the expected digit. Replace the last character with that digit to make the number valid (if the rest of the number is correct).

Validation is not proof of existence. A valid ISBN only means the format and check digit are correct. It does not prove the book exists or that the number is assigned to a real title. Use it for format and typo checks, not for authority.

Privacy. Do not rely on the optional book-details feature for sensitive use; the ISBN may be sent for lookup.