Bookdigit

A small check for a copied number

Does the final digit add up?

Check an ISBN-13 checksum and see the arithmetic. Your entry stays in this browser. A matching check digit is one check on a transcription; it does not identify or verify a book.

Enter all 13 digits, with optional plain hyphens or spaces. Omit the word ISBN. Supports 978 and 979, except the 979-0 music prefix.

Editing clears the previous diagnostic and disables its exports. Choose Check checksum again after an edit.

A checksum is a consistency check

The first twelve digits get alternating weights of 1 and 3. Their weighted sum determines the thirteenth digit. Bookdigit shows that sum rather than hiding the method behind a badge.

If the checksum does not match, the tool tells you which final digit the first twelve digits would require. It cannot tell whether those twelve digits were copied correctly. Compare the whole identifier with the source before changing it.

Read the method and boundaries or work through an example.