---
title: "What Is an ISBN? Book Numbers for KDP Authors"
description: "What an ISBN is, what its 13 digits mean, which formats need one on KDP, free KDP ISBNs versus your own, and the imprint rules that trip people up."
url: "https://www.superebook.ai/glossary/isbn"
updated: "2026-10-20"
---

# ISBN

**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 October 20, 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.

## Questions

**Does an ISBN protect my copyright?** No. The International ISBN Agency says an ISBN is an identifier and does not convey any form of legal or copyright protection.

**Does a Kindle ebook need an ISBN?** Not on KDP. Its help page says ebooks and low-content books are the only formats where an ISBN is optional; Amazon identifies the ebook by its ASIN.

**Should I use a free KDP ISBN?** It works if the print edition will only ever be sold through KDP. Its imprint shows as "Independently published", and the number can't be used with another publishing service.

## Sources

- [International ISBN Agency: What is an ISBN?](https://www.isbn-international.org/content/what-isbn/10): The 13 digits and five elements, what an ISBN identifies, a separate ISBN per product form, no copyright protection, and who applies.
- [KDP: What is an ISBN and Imprint?](https://kdp.amazon.com/en_US/help/topic/G201834170): Each print format needs its own ISBN; optional for ebooks and low-content books; imprint matching, case and spacing; no placeholders.
- [KDP: Get an ISBN](https://kdp.amazon.com/en_US/help/topic/GTJ8LBXL6Z4WV5QX): Free KDP ISBNs registered with Bowker, paperback and hardcover only, usable only on KDP; low-content books not eligible.

## Related terms

- [ASIN](https://www.superebook.ai/glossary/asin.md)
- [Low-content book](https://www.superebook.ai/glossary/low-content-book.md)
- [Expanded Distribution](https://www.superebook.ai/glossary/expanded-distribution.md)
- [BISAC subject codes](https://www.superebook.ai/glossary/bisac-subject-codes.md)

## Read next

- [Do you need an ISBN for an ebook on KDP?](https://www.superebook.ai/guides/do-you-need-an-isbn-for-an-ebook.md)
- [KDP vs Gumroad vs Etsy for selling ebooks](https://www.superebook.ai/guides/kdp-vs-gumroad-vs-etsy.md)
- [What is KDP Expanded Distribution?](https://www.superebook.ai/guides/kdp-expanded-distribution.md)
- [All glossary terms](https://www.superebook.ai/glossary.md)