About this tool

A small, inspectable number check

Bookdigit is an independent free calculator for one task: checking the arithmetic of an entered ISBN-13 checksum. Its result includes the normalized digits, the supplied and expected final digits, and each weighted product so the calculation can be followed by hand.

It is useful when checking a copied identifier or learning how a check digit works. It is not a book catalog, publisher registry, agency service or allocation system. It has no affiliation with the International ISBN Agency or the Library of Congress.

Free to use

No account, subscription or payment is required. This release has no active advertising, analytics or consent-platform loader. Advertising may support the site in a later configured release; no paid result or allocation service is offered here.

Keep the result in context

Checksum agreement is narrower than a valid assignment. Some errors can escape a checksum, and a mismatch does not identify which character was copied incorrectly. Use the original source or an appropriate catalog for questions about an actual book. See the method and limitations for the exact input rules.

To report a calculation problem, use the shared public contact address. Supplying a small reproducible example is optional; never send credentials or payment details.