ISBN Defined.
Definition
An ISBN (International Standard Book Number) is a 13-digit identifier for one edition and format of a book, used by publishers, stores and libraries for ordering, listing and stock control. Each product form, such as a paperback, a hardcover or an EPUB, should have its own. On KDP, print books need one and ebooks don't.
Sources checked 20 October 2026.
How it works
Since 2007 every ISBN has 13 digits in five parts: a prefix (978 or 979), a registration group for the country or language area, a registrant element for the publisher, a publication element for the edition and format, and a check digit. National agencies issue them, Bowker in the US, and it is the publisher who applies, which can be you if you publish yourself. An ISBN is an identifier only: it gives no copyright protection. On KDP a paperback or hardcover can use a free KDP ISBN, registered with Bowker and showing the imprint "Independently published", or one you bought, which must match the title, author and imprint you registered.
An example
You publish a paperback with a free KDP ISBN and later want the same paperback printed and sold by another service. The number can't go with it: KDP says its free ISBNs can only be used on KDP. If you plan to sell a print edition elsewhere, buy your own ISBN from your national agency and register it under your imprint before you publish anywhere.
Common mistakes
Reusing one ISBN across the paperback and the hardcover, or across a revised edition with significant changes. Typing the imprint slightly differently from the agency's record: KDP checks it, and says even a trailing space can cause a mismatch. Entering placeholders such as "n/a". And expecting a free KDP ISBN on a low-content book, which isn't eligible.
