An ISBN decoder reveals the hidden structure inside a book's International Standard Book Number. Every ISBN encodes a registration group (language/country), a registrant (publisher), and a publication identifier — along with a check digit that catches transcription errors. This decoder handles both ISBN-10 and ISBN-13 formats.
Enter ISBN-10 or ISBN-13
Hyphens and spaces are ignored. Both ISBN-10 and ISBN-13 are accepted. Decodes as you type.
ISBN Structure
How to Use the ISBN Decoder
The International Standard Book Number (ISBN) is a 10 or 13 digit identifier assigned to every commercially published book. Far from random, each segment of an ISBN identifies the language group, publisher, and specific title. This free ISBN decoder reveals each component and validates the check digit.
Step 1: Enter the ISBN
Type or paste the ISBN in the input field. You can include or omit hyphens — they are automatically stripped. The decoder accepts ISBN-10 (10 digits, may end in 'X'), ISBN-13 (13 digits, starts with 978 or 979), and hyphenated formats like 978-0-306-40615-7.
Step 2: Understand the Registration Group
The registration group element identifies the country or language area of the publisher. Group 0 and 1 cover English-language publishers worldwide (US, UK, Australia, Canada, etc.). Group 2 is French, 3 is German, 4 is Japan, 5 is Russia/former USSR, and 7 is China. Higher numbers (80-94 and above) identify specific smaller countries or specialized publisher groups.
Step 3: Read the Publisher and Title Identifiers
After the registration group, the registrant element identifies the specific publisher. Publishers pay for an ISBN registrant range — a small publisher might have a longer registrant number (fewer titles), while a major publisher gets a shorter registrant prefix (room for thousands of titles). The publication element then identifies the specific title within that publisher's catalog.
Step 4: Verify the Check Digit
For ISBN-13, the check digit is calculated by multiplying each of the first 12 digits alternately by 1 and 3, summing the products, and computing (10 minus the sum modulo 10) modulo 10. For ISBN-10, each digit is multiplied by its position (1 through 9), the products are summed, and the remainder when divided by 11 is the check digit (10 becomes 'X'). A mismatch means a typo somewhere in the ISBN.
Converting Between ISBN-10 and ISBN-13
The decoder automatically shows both the ISBN-10 and ISBN-13 equivalents. To convert ISBN-10 to ISBN-13, prepend '978' and recalculate the check digit. Note that ISBNs starting with '979' have no ISBN-10 equivalent — the 979 prefix was added to expand the ISBN space after 978 prefixes became exhausted in some registration groups.
FAQ
What does an ISBN decoder do?
An ISBN decoder breaks down a book's International Standard Book Number into its component parts: prefix (ISBN-13 only), registration group (language/country), registrant (publisher), publication (title identifier), and check digit. It also validates the check digit to confirm the ISBN is correct.
Is this ISBN lookup free?
Yes, this ISBN decoder is completely free. Enter any ISBN-10 or ISBN-13 and instantly see the full structural breakdown — no signup or account required.
Is my data safe?
Yes, all ISBN decoding happens entirely in your browser. No data is sent to any server. The tool works offline once the page loads.
What is the difference between ISBN-10 and ISBN-13?
ISBN-10 was the original 10-digit format used before 2007. ISBN-13 is the current standard — it adds a 3-digit GS1 prefix (978 or 979) to the 9-digit ISB number and uses a different check digit calculation (mod-10 weighted sum instead of mod-11). All books published after 2007 use ISBN-13.
How is the ISBN check digit calculated?
For ISBN-13: multiply alternating digits by 1 and 3, sum all products, and the check digit is (10 minus the remainder when divided by 10), modulo 10. For ISBN-10: multiply each digit by its position (1 through 9), sum the products, divide by 11 — the remainder is the check digit (10 = 'X').
What do the registration group numbers mean?
The registration group identifies the language/country of publication. Group 0 and 1 are English-speaking countries, 2 is French, 3 is German, 4 is Japan, 5 is Russia, 7 is China, and numbers 80-94 and higher cover various specific countries and regional publishers.
Can I convert ISBN-10 to ISBN-13?
Yes — to convert ISBN-10 to ISBN-13, prefix it with '978', then recalculate the check digit using the ISBN-13 algorithm. This decoder performs the conversion automatically and displays both formats for any valid ISBN-10.