ISBN Validator

What this tool does

The ISBN Validator accepts an ISBN-10 or ISBN-13, with or without hyphens and spaces, strips the separators, detects which format you typed, and validates the check digit using the official ISO 2108:2017 arithmetic. For ISBN-10 it runs a weighted sum with weights 10 through 1 and requires the total to be divisible by 11, accepting an X in the check-digit position to represent the value 10. For ISBN-13 it runs a weighted sum with alternating weights 1 and 3 and requires the total to be divisible by 10. The tool then shows the per-digit weighted-sum working in a table so you can see exactly why a number passes or fails, and offers a round-trip conversion between ISBN-10 and ISBN-13 where one is defined. Everything happens in the browser; no input is uploaded, no API is called, and no analytics leaves your machine.

How to use

1. Type or paste an ISBN-10 or ISBN-13 into the input box. Hyphens, spaces, and other whitespace are tolerated and ignored.
2. The tool strips the separators automatically and reports the detected format (ISBN-10 or ISBN-13 / Bookland / EAN-13).
3. Read the result banner: VALID means the check digit matches the weighted sum, INVALID means it does not. The weighted sum and the modulus remainder are shown beneath.
4. Read the per-digit table to see the digit, its weight, and the product for every position. This is the working that proves the result.
5. If your ISBN-10 is valid, the tool computes the matching ISBN-13. If your 978-prefixed ISBN-13 is valid, the tool computes the matching ISBN-10. 979-prefixed ISBN-13s cannot be reversed because the 979 namespace was introduced after ISBN-10 was retired.
6. Use the formatted output (with standard group hyphens when the prefix is recognised) for catalogues, library records, and metadata.

How the check digits work

ISBN-10 (mod 11)

Take the first nine digits as d1 through d9 and the check digit as c10. Multiply each digit by a weight that runs from 10 down to 1, so the first digit is weighted by 10, the second by 9, and so on, with the check digit weighted by 1. Add all ten products. The ISBN is valid if and only if the total is divisible by 11, that is, total mod 11 equals 0. Because the weights are large and coprime with 11, this catches almost every single-digit typo and almost every transposition of two adjacent digits, both common data-entry errors. The check digit c10 can be 0 through 9 or the letter X, which represents the value 10. That is the only position where X is legal; anywhere else, an X is a typo and the validator will reject it.

ISBN-13 and EAN-13 (mod 10)

Take the first twelve digits as d1 through d12 and the check digit as c13. Multiply d1 by 1, d2 by 3, d3 by 1, d4 by 3, and so on, alternating across the full thirteen positions. Add all thirteen products. The ISBN is valid if and only if the total is divisible by 10, that is, total mod 10 equals 0. The check digit is chosen to make that hold. Notice that the weight pattern is 1, 3, 1, 3, ..., which is exactly the same as a standard EAN-13 retail barcode check, because ISBN-13 is an EAN-13 with the 978 or 979 Bookland prefix.

Why the check digit can be X

ISBN-10 uses modulus 11, so the check digit has eleven possible values: 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, and 10. There is no Arabic numeral for ten, so the publishing industry chose the letter X by convention. Roman numerals were already familiar to typesetters and librarians, and X is visually distinct from any digit. ISBN-13 uses modulus 10, so the check digit is always a single digit 0 through 9 and X is illegal anywhere in an ISBN-13.

The 978 and 979 Bookland prefix

When ISBNs were migrated to ISBN-13 on 1 January 2007, the new numbers had to fit inside the existing EAN-13 barcode standard so they could ride on the same retail infrastructure. The International ISBN Agency reserved the 978 prefix, the so-called Bookland code, for existing ISBNs. As the ISBN namespace started to run out in the late 2000s, a second prefix, 979, was added in 2007 for newly assigned ISBNs. Any 13-digit number whose prefix is 978 or 979 is an ISBN; any other prefix means the number is a valid EAN-13 (it will pass the mod-10 check) but it is not a book identifier.

The five ISBN element groups

Every fully hyphenated ISBN has five pieces: the GS1 prefix element (978 or 979 for ISBN-13), the registration group element (a country or language group such as 0 for English-speaking countries or 2 for French), the registrant element (a specific publisher), the publication element (a specific title or edition from that publisher), and the check digit. The lengths of the last three vary by group and registrant. Group 0 has one digit, group 2 has one digit, group 7 (China) has one digit, while group 950 or 951 has three digits. The publisher and title lengths then make up the remainder up to the check digit. This tool looks up the registered range table and inserts the hyphens at the correct boundaries; when the prefix is not in the table, it falls back to the unhyphenated form so you never see a misleading hyphenation.

Worked examples

1. ISBN-10 0306406152 (canonical example). Weights 10 down to 1. 0·10 = 0, 3·9 = 27, 0·8 = 0, 6·7 = 42, 4·6 = 24, 0·5 = 0, 6·4 = 24, 1·3 = 3, 5·2 = 10, 2·1 = 2. Sum = 0+27+0+42+24+0+24+3+10+2 = 132. 132 mod 11 = 0, so this ISBN-10 is VALID. The matching ISBN-13 is computed by prefixing 978, dropping the old check, and recomputing mod-10 over the 12 digits 978030640615: the result is 9780306406157, sum = 1·1 + 3·3 + 8·1 + 0·3 + 3·1 + 0·3 + 6·1 + 4·3 + 0·1 + 6·3 + 1·1 + 5·3 = 1+9+8+0+3+0+6+12+0+18+1+15 = 73. C13 must satisfy total mod 10 = 0, so c13 = (10 - 73 mod 10) mod 10 = (10 - 3) mod 10 = 7. Result: 9780306406157. VALID.

2. ISBN-10 043942089X (ends in X). Stripped: 043942089X. Weights 10..1. 0·10 = 0, 4·9 = 36, 3·8 = 24, 9·7 = 63, 4·6 = 24, 2·5 = 10, 0·4 = 0, 8·3 = 24, 9·2 = 18, X·1 = 10·1 = 10. Sum = 0+36+24+63+24+10+0+24+18+10 = 209. 209 mod 11 = 0 (because 11·19 = 209). VALID.

3. ISBN-13 9780306406157. Weights 1,3,1,3,...,1 across 13 positions. Products: 9·1=9, 7·3=21, 8·1=8, 0·3=0, 3·1=3, 0·3=0, 6·1=6, 4·3=12, 0·1=0, 6·3=18, 1·1=1, 5·3=15, 7·1=7. Sum = 9+21+8+0+3+0+6+12+0+18+1+15+7 = 100. 100 mod 10 = 0. VALID.

4. ISBN-10 0306406153 (deliberately wrong check digit). Weights 10..1, products 0, 27, 0, 42, 24, 0, 24, 3, 10, 3·1 = 3. Sum = 133. 133 mod 11 = 1, not 0. INVALID. The tool will also tell you what the correct check digit would have been: 2, because 132 is the multiple of 11 closest to 133.

5. ISBN-13 9791036206474 (979 prefix). Weights 1,3,1,3,...,1. Products: 9, 21, 9, 3, 0, 18, 3, 6, 2, 18, 0, 12, 16. Sum = 9+21+9+3+0+18+3+6+2+18+0+12+16 = 117. 117 mod 10 = 7, not 0. INVALID. To produce a valid 979-prefixed ISBN-13 the check digit must satisfy (sum_without_check + c13) mod 10 = 0, so c13 = (10 - 7) mod 10 = 3 and the corrected number is 9791036206473.

6. ISBN-13 -> ISBN-10 reverse (978 prefix only). Given 9780306406157, drop the 978 prefix to get 030640615 and compute the mod-11 check: 0·10+3·9+0·8+6·7+4·6+0·5+6·4+1·3+5·2 = 0+27+0+42+24+0+24+3+10 = 130. c10 = (11 - 130 mod 11) mod 11 = (11 - 9) mod 11 = 2. So 9780306406157 reverses to 0306406152. VALID.

Where it shows up

Library systems. Every library catalogue, from a small public library to the Library of Congress, stores ISBNs as the primary key for books. Library staff and patrons routinely mistype a digit or two, and check-digit validation is the first line of defence against bad records entering the catalogue. The same arithmetic is built into circulation systems, inter-library loan systems, and discovery layers.

Retail barcodes. Modern ISBN-13s are printed as EAN-13 barcodes on the back cover of every book. The same barcode format is used for groceries, hardware, and pharmaceuticals, which is why a supermarket scanner can read a book, the barcode does not know or care that the number is a Bookland prefix. Retailers rely on the mod-10 check to detect scanning errors before they become inventory errors.

Publishing metadata. ONIX (Online Information Exchange), the standard metadata format for the book supply chain, carries ISBN-13 as one of its primary product identifiers. Publishers, distributors, and retailers all run mod-10 validation on incoming ONIX feeds so that bad data is rejected at the boundary rather than propagating through the supply chain.

Data validation in book APIs. Open library APIs (OpenLibrary, Google Books, ISBNdb, WorldCat) all accept ISBNs and return metadata. Client code that calls those APIs almost always validates the check digit first to avoid an obvious typo turning into a confusing "not found" response from a server that actually had the record.

Common mistakes

Confusing ISBN-13 with generic EAN-13. A 13-digit barcode on a book is genuinely an EAN-13 and genuinely an ISBN-13, because all current ISBNs ride the EAN-13 standard. But the reverse is not true: a tin of beans has a valid EAN-13 with no relationship to books. Validating only the mod-10 check digit tells you the digits are well-formed, not that they belong to a book.

Dropping the leading zero. ISBN-10s frequently start with 0 (the English-language registration group). When exporting to systems that treat ISBN as a number rather than a string, older spreadsheets, SQL INTEGER columns, naive CSV importers, the leading zero is silently stripped and the result becomes 9 digits long. Always store ISBNs as text.

Assuming a valid check digit means the book exists. This is the single most consequential mistake. A correctly formatted ISBN-13 only proves that the thirteen digits satisfy the mod-10 weighted-sum rule. It does not prove the ISBN was ever assigned, that it is in print, that the title you typed is correct, or that the publisher exists. Many perfectly valid check digits correspond to ISBNs that have never been issued, or to ISBNs that have been issued and then withdrawn. If you need to verify that a book is real and in print, you must consult an authoritative registry such as the ISBN Agency's directory or a book database, not the arithmetic alone.

Mixing up the 978 and 979 prefixes. Both are valid ISBN-13 prefixes, but only 978-prefixed numbers have a corresponding ISBN-10. If your conversion tool reports "no ISBN-10 equivalent" for a 979-prefixed ISBN-13, that is correct behaviour, the ISBN-10 namespace was retired before 979 was introduced.

Treating X as interchangeable with the digit 10 in ISBN-10. X means the value 10 in the final position of an ISBN-10 and nowhere else. It is also a literal X in the data: when you store the ISBN, store the X, do not convert it to 10, do not lowercase it, and do not silently drop it.

Frequently Asked Questions

What is the difference between ISBN-10 and ISBN-13?

ISBN-10 is the pre-2007 format with ten characters and a mod-11 check digit that can be X. ISBN-13 is the post-2007 format with thirteen digits and a mod-10 check digit that is always a digit. Every ISBN-10 has a matching ISBN-13 with the 978 Bookland prefix, but not every ISBN-13 (in particular, any 979-prefixed number) has a matching ISBN-10.

Why does my ISBN-13 have a 979 prefix instead of 978?

The 978 prefix was the original Bookland code assigned when ISBN-10 was migrated to ISBN-13. When that namespace started to run out, the International ISBN Agency introduced 979 in 2007 for newly assigned ISBNs. A 979 ISBN-13 is no less valid than a 978 one, it simply has no ISBN-10 equivalent because ISBN-10 was retired before 979 was introduced.

Does a valid check digit mean the ISBN was assigned to a real book?

No. A valid check digit only proves that the digits are arithmetically well-formed. Many valid check digits correspond to ISBNs that have never been issued, or to ISBNs that have been issued and then withdrawn. To confirm a book exists, consult the ISBN Agency directory, a library catalogue, or a book database such as OpenLibrary.

Why is X sometimes a valid ISBN character?

ISBN-10 uses mod 11, so the check digit has eleven possible values from 0 to 10. There is no Arabic numeral for 10, so publishing convention uses X to represent the value 10 in the check-digit position only. ISBN-13 uses mod 10 and the check digit is always a single digit, so X is illegal in an ISBN-13.

Can I convert an ISBN-13 back to an ISBN-10?

Only if the ISBN-13 starts with 978. Drop the 978 prefix to get nine digits, multiply by weights 10 down to 2, and choose the mod-11 check digit (which may be X). 979-prefixed ISBN-13s cannot be reversed because ISBN-10 was retired before 979 was introduced.

Is an ISBN-13 the same as an EAN-13?

ISBN-13 is a subset of EAN-13 whose prefix is 978 or 979 (the Bookland range). Every ISBN-13 is a valid EAN-13 because it satisfies the same mod-10 check, but most EAN-13s are not ISBNs because their prefix is something other than 978 or 979.

Is the validator safe for confidential catalogues?

Yes. The validator runs entirely in your browser, there are no network calls, no uploads, no telemetry. You can validate embargoed ISBNs, internal catalogue numbers, and customer-supplied data without any of it leaving your machine.

References

• ISO 2108:2017, Information and documentation, International Standard Book Number (ISBN). The official standard that defines the ISBN structure, element groups, and check-digit rules for ISBN-13. The ISBN-10 rules in this tool match the historical pre-2007 standard (sometimes referred to as ISO 2108:1992 or earlier editions), which is still useful for legacy catalogues.
• International ISBN Agency, ISBN Users' Manual. The operational companion to ISO 2108, published by the agency that administers the ISBN namespace. It documents the registration group ranges, the assignment rules for publishers, and the migration from ISBN-10 to ISBN-13 on 1 January 2007.
• GS1, General Specifications. The standard that defines EAN-13 barcodes and the mod-10 check digit they share with ISBN-13. Published by GS1, the body that maintains retail barcode standards worldwide. ISBN-13 is registered as an EAN-13 with the 978 or 979 Bookland prefix.

A valid check digit proves only that the digits are well-formed, not that the ISBN was ever assigned to a real book.