Free ISBN Check Digit Calculator

Calculate and verify ISBN check digits instantly for both ISBN-10 and ISBN-13 formats. Our free online tool automatically detects your ISBN format and applies the correct validation algorithm to ensure accuracy.

Perfect for publishers, authors, librarians, and booksellers who need to verify ISBN accuracy or calculate missing check digits. Supports both single and bulk processing.

Calculate Your ISBN Check Digit

Enter your ISBN number without the check digit below. The tool automatically detects whether you're using ISBN-10 (9 digits) or ISBN-13 (12 digits) format and calculates the appropriate check digit.

Do NOT generate ISBNs using a calculator. The check digit tool is for verification/correction only—unregistered ISBNs are invalid and may cause legal/distribution problems.

Why Use Our ISBN Check Digit Calculator?

100% Free

No registration or payment required

Auto-Detection

Automatically identifies ISBN-10 or ISBN-13

Bulk Processing

Calculate multiple ISBNs at once

Instant Results

Get accurate check digits immediately

Error Detection

Validates ISBN integrity and format

Industry Standard

Uses official ISBN calculation algorithms

ISBN check digit calculator for book validation and verification

Understanding ISBN Check Digits

The check digit is the last digit of an ISBN and is used to validate the integrity of the ISBN number. It helps detect errors in the ISBN that might occur during manual entry or scanning.

Learn how to calculate check digit manually:

How Check Digits Work

The check digit is calculated using a mathematical formula:

For ISBN-10:

Multiply each digit by its position (1 to 9)
Sum the products
Divide the sum by 11
The check digit is 11 minus the remainder (if 10, use 'X')

For ISBN-13:

Alternate between multiplying digits by 1 and 3
Sum the products
The check digit is 10 minus (sum modulo 10)

Frequently Asked Questions About ISBN Check Digits

What is a check digit?

A check digit is a form of redundancy check used for error detection on identification numbers, such as ISBNs. It is the last digit of the ISBN and is calculated from the other digits using a mathematical formula to validate the integrity of the ISBN number.

Why is the check digit important?

The check digit helps detect common errors in manual data entry or scanning. It can identify:

Single digit errors (changing one digit)
Transposition errors (swapping two adjacent digits)
Twin errors (changing two digits to their sum or difference)

Can the check digit be 'X'?

Yes, but only in ISBN-10. When the check digit calculation results in 10, the character 'X' is used to represent it. This never occurs in ISBN-13, where check digits are always numeric (0-9).

What if I get the wrong check digit?

If your calculated check digit doesn't match the one on your book, either:

The ISBN is invalid or incorrect
There was an error in entering the other digits
The check digit on the book is wrong (rare but possible)

How is the ISBN-10 check digit calculated?

For ISBN-10, multiply each digit by its position (1 to 9), sum the products, divide the sum by 11, and the check digit is 11 minus the remainder. If the result is 10, use 'X'.

How is the ISBN-13 check digit calculated?

For ISBN-13, alternate between multiplying digits by 1 and 3, sum the products, and the check digit is 10 minus (sum modulo 10). The result is always a single digit from 0-9.

Can I use this tool to generate ISBNs?

No, do not generate ISBNs using a calculator. The check digit tool is for verification and correction only. Unregistered ISBNs are invalid and may cause legal and distribution problems. ISBNs must be officially registered through authorized agencies.

Does this tool work for both ISBN-10 and ISBN-13?

Yes, the tool automatically detects whether you're entering an ISBN-10 (9 digits) or ISBN-13 (12 digits) and applies the appropriate calculation formula for each format.