ISBN Check Digit Calculator
Calculate the check digit for ISBN-10 or ISBN-13. Enter the first 9 digits for ISBN-10 or the first 12 digits for ISBN-13.
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:
ISBN-10 Check Digit Calculation
ISBN-13 Check Digit Calculation
ISBN Validator Tool
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
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.
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)