International Standard Book Number

ISBN issuance is country-specific, in that ISBNs are issued by the ISBN registration agency that is responsible for that country or territory regardless of the publication language. The ranges of ISBNs assigned to any particular country are based on the publishing profile of the country concerned, and so the ranges will vary depending on the number of books and the number, type, and size of publishers that are active. Some ISBN registration agencies are based in national libraries or within ministries of culture and thus may receive direct funding from government to support their services. In other cases, the ISBN registration service is provided by organisations such as bibliographic data providers that are not government funded.[17]

A full directory of ISBN agencies is available on the International ISBN Agency website.[18] List for a few countries is given below:

• Australia – Thorpe-Bowker[19][20]
• Brazil – The National Library of Brazil;[21] (Up to 28 February 2020)[22]
• Brazil – Câmara Brasileira do Livro[23] (From 1 March 2020)[22]
• Canada – English Library and Archives Canada, a government agency; French Bibliothèque et Archives nationales du Québec;
• Colombia – Cámara Colombiana del Libro, an NGO
• Hong Kong – Books Registration Office (BRO), under the Hong Kong Public Libraries[24]
• India – The Raja Rammohun Roy National Agency for ISBN (Book Promotion and Copyright Division), under Department of Higher Education, a constituent of the Ministry of Human Resource Development[25]
• Iceland – Landsbókasafn (National and University Library of Iceland)
• Israel – The Israel Center for Libraries[26]
• Italy – EDISER srl, owned by Associazione Italiana Editori (Italian Publishers Association)[27][28]
• Maldives – The National Bureau of Classification (NBC)
• Malta – The National Book Council (Maltese: Il-Kunsill Nazzjonali tal-Ktieb)[29][30][31]
• Morocco – The National Library of Morocco
• New Zealand – The National Library of New Zealand[32]
• Pakistan – National Library of Pakistan
• Philippines – National Library of the Philippines[33]
• South Africa – National Library of South Africa
• Spain – Spanish ISBN Agency – Agencia del ISBN
• Turkey – General Directorate of Libraries and Publications, a branch of the Ministry of Culture[34]
• United Kingdom and Republic of Ireland – Nielsen Book Services Ltd, part of Nielsen Holdings N.V.[35]
• United States – R. R. Bowker[6][36]

The ISBN registration group identifier is a 1- to 5-digit number that is valid within a single prefix element (i.e. one of 978 or 979),[15]^:11 and can be separated between hyphens, such as "978-1-...". Registration group identifiers have primarily been allocated within the 978 prefix element.[37] The single-digit group identifiers within the 978-prefix element are: 0 or 1 for English-speaking countries; 2 for French-speaking countries; 3 for German-speaking countries; 4 for Japan; 5 for Russian-speaking countries; and 7 for People's Republic of China. An example 5-digit group identifier is 99936, for Bhutan. The allocated group IDs are: 0–5, 600–625, 65, 7, 80–94, 950–989, 9917–9989, and 99901–99983.[38] Books published in rare languages typically have longer group identifiers.[39]

Within the 979 prefix element, the registration group identifier 0 is reserved for compatibility with International Standard Music Numbers (ISMNs), but such material is not actually assigned an ISBN.[40] The registration group identifiers within prefix element 979 that have been assigned are 8 for the United States of America, 10 for France, 11 for the Republic of Korea, and 12 for Italy.[41]

The original 9-digit standard book number (SBN) had no registration group identifier, but prefixing a zero (0) to a 9-digit SBN creates a valid 10-digit ISBN.

The national ISBN agency assigns the registrant element (cf. Category:ISBN agencies) and an accompanying series of ISBNs within that registrant element to the publisher; the publisher then allocates one of the ISBNs to each of its books. In most countries, a book publisher is not legally required to assign an ISBN, although most large bookstores only handle publications that have ISBNs assigned to them.[42][43][44]

A listing of more than 900,000 assigned publisher codes is published, and can be ordered in book form. The web site of the ISBN agency does not offer any free method of looking up publisher codes.[45] Partial lists have been compiled (from library catalogs) for the English-language groups: identifier 0 and identifier 1.

Publishers receive blocks of ISBNs, with larger blocks allotted to publishers expecting to need them; a small publisher may receive ISBNs of one or more digits for the registration group identifier, several digits for the registrant, and a single digit for the publication element. Once that block of ISBNs is used, the publisher may receive another block of ISBNs, with a different registrant element. Consequently, a publisher may have different allotted registrant elements. There also may be more than one registration group identifier used in a country. This might occur once all the registrant elements from a particular registration group have been allocated to publishers.

By using variable block lengths, registration agencies are able to customise the allocations of ISBNs that they make to publishers. For example, a large publisher may be given a block of ISBNs where fewer digits are allocated for the registrant element and many digits are allocated for the publication element; likewise, countries publishing many titles have few allocated digits for the registration group identifier and many for the registrant and publication elements.[46] Here are some sample ISBN-10 codes, illustrating block length variations.

English-language registration group elements are 0 and 1 (2 of more than 220 registration group elements). These two registration group elements are divided into registrant elements in a systematic pattern, which allows their length to be determined, as follows:[47]

According to the 2001 edition of the International ISBN Agency's official user manual,[48] the ISBN-10 check digit (which is the last digit of the 10-digit ISBN) must range from 0 to 10 (the symbol 'X' is used for 10), and must be such that the sum of the ten digits, each multiplied by its (integer) weight, descending from 10 to 1, is a multiple of 11. That is, if x_i is the ith digit, then x₁₀ must be chosen such that:

${\displaystyle \sum _{i=1}^{10}(11-i)x_{i}\equiv 0{\pmod {11}}}$

For example, for an ISBN-10 of 0-306-40615-2:

${\displaystyle {\begin{aligned}s&=(0\times 10)+(3\times 9)+(0\times 8)+(6\times 7)+(4\times 6)+(0\times 5)+(6\times 4)+(1\times 3)+(5\times 2)+(2\times 1)\\&=0+27+0+42+24+0+24+3+10+2\\&=132=12\times 11\end{aligned}}}$

Formally, using modular arithmetic, this is rendered:

${\displaystyle (10x_{1}+9x_{2}+8x_{3}+7x_{4}+6x_{5}+5x_{6}+4x_{7}+3x_{8}+2x_{9}+x_{10})\equiv 0{\pmod {11}}.}$

It is also true for ISBN-10s that the sum of all ten digits, each multiplied by its weight in ascending order from 1 to 10, is a multiple of 11. For this example:

${\displaystyle {\begin{aligned}s&=(0\times 1)+(3\times 2)+(0\times 3)+(6\times 4)+(4\times 5)+(0\times 6)+(6\times 7)+(1\times 8)+(5\times 9)+(2\times 10)\\&=0+6+0+24+20+0+42+8+45+20\\&=165=15\times 11\end{aligned}}}$

Formally, this is rendered:

${\displaystyle (x_{1}+2x_{2}+3x_{3}+4x_{4}+5x_{5}+6x_{6}+7x_{7}+8x_{8}+9x_{9}+10x_{10})\equiv 0{\pmod {11}}.}$

The two most common errors in handling an ISBN (e.g. when typing it or writing it down) are a single altered digit or the transposition of adjacent digits. It can be proven mathematically that all pairs of valid ISBN-10s differ in at least two digits. It can also be proven that there are no pairs of valid ISBN-10s with eight identical digits and two transposed digits. (These proofs are true because the ISBN is less than eleven digits long and because 11 is a prime number.) The ISBN check digit method therefore ensures that it will always be possible to detect these two most common types of error, i.e., if either of these types of error has occurred, the result will never be a valid ISBN – the sum of the digits multiplied by their weights will never be a multiple of 11. However, if the error were to occur in the publishing house and remain undetected, the book would be issued with an invalid ISBN.[49]

In contrast, it is possible for other types of error, such as two altered non-transposed digits, or three altered digits, to result in a valid ISBN (although it is still unlikely).

Each of the first nine digits of the 10-digit ISBN—excluding the check digit itself—is multiplied by its (integer) weight, descending from 10 to 2, and the sum of these nine products found. The value of the check digit is simply the one number between 0 and 10 which, when added to this sum, means the total is a multiple of 11.

For example, the check digit for an ISBN-10 of 0-306-40615-? is calculated as follows:

${\displaystyle {\begin{aligned}s&=(0\times 10)+(3\times 9)+(0\times 8)+(6\times 7)+(4\times 6)+(0\times 5)+(6\times 4)+(1\times 3)+(5\times 2)\\&=130\end{aligned}}}$

Adding 2 to 130 gives a multiple of 11 (because 132 = 12×11) – this is the only number between 0 and 10 which does so. Therefore, the check digit has to be 2, and the complete sequence is ISBN 0-306-40615-2. If the value of ${\displaystyle x_{10}}$ required to satisfy this condition is 10, then an 'X' should be used.

Alternatively, modular arithmetic is convenient for calculating the check digit using modulus 11. The remainder of this sum when it is divided by 11 (i.e. its value modulo 11), is computed. This remainder plus the check digit must equal either 0 or 11. Therefore, the check digit is (11 minus the remainder of the sum of the products modulo 11) modulo 11. Taking the remainder modulo 11 a second time accounts for the possibility that the first remainder is 0. Without the second modulo operation, the calculation could result in a check digit value of 11−0 = 11, which is invalid. (Strictly speaking, the first "modulo 11" is not needed, but it may be considered to simplify the calculation.)

For example, the check digit for the ISBN-10 of 0-306-40615-? is calculated as follows:

${\displaystyle {\begin{aligned}s&=(11-(((0\times 10)+(3\times 9)+(0\times 8)+(6\times 7)+(4\times 6)+(0\times 5)+(6\times 4)+(1\times 3)+(5\times 2))\,{\bmod {\,}}11))\,{\bmod {\,}}11\\&=(11-((0+27+0+42+24+0+24+3+10)\,{\bmod {\,}}11))\,{\bmod {\,}}11\\&=(11-((130)\,{\bmod {\,}}11))\,{\bmod {\,}}11\\&=(11-(9))\,{\bmod {\,}}11\\&=(2)\,{\bmod {\,}}11\\&=2\end{aligned}}}$

Thus the check digit is 2.

It is possible to avoid the multiplications in a software implementation by using two accumulators. Repeatedly adding t into s computes the necessary multiples:

// Returns ISBN error syndrome, zero for a valid ISBN, non-zero for an invalid one.
// digits[i] must be between 0 and 10.
int CheckISBN(int const digits[10])
{
        int i, s = 0, t = 0;

        for (i = 0; i < 10; i++) {
                t += digits[i];
                s += t;
        }
        return s % 11;
}

The modular reduction can be done once at the end, as shown above (in which case s could hold a value as large as 496, for the invalid ISBN 99999-999-9-X), or s and t could be reduced by a conditional subtract after each addition.

Appendix 1 of the International ISBN Agency's official user manual[15]^:33 describes how the 13-digit ISBN check digit is calculated. The ISBN-13 check digit, which is the last digit of the ISBN, must range from 0 to 9 and must be such that the sum of all the thirteen digits, each multiplied by its (integer) weight, alternating between 1 and 3, is a multiple of 10. As ISBN-13 is a subset of EAN-13, the algorithm for calculating the check digit is exactly the same for both.

Formally, using modular arithmetic, this is rendered:

${\displaystyle (x_{1}+3x_{2}+x_{3}+3x_{4}+x_{5}+3x_{6}+x_{7}+3x_{8}+x_{9}+3x_{10}+x_{11}+3x_{12}+x_{13})\equiv 0{\pmod {10}}.}$

The calculation of an ISBN-13 check digit begins with the first twelve digits of the 13-digit ISBN (thus excluding the check digit itself). Each digit, from left to right, is alternately multiplied by 1 or 3, then those products are summed modulo 10 to give a value ranging from 0 to 9. Subtracted from 10, that leaves a result from 1 to 10. A zero (0) replaces a ten (10), so, in all cases, a single check digit results.

For example, the ISBN-13 check digit of 978-0-306-40615-? is calculated as follows:

s = 9×1 + 7×3 + 8×1 + 0×3 + 3×1 + 0×3 + 6×1 + 4×3 + 0×1 + 6×3 + 1×1 + 5×3
  =   9 +  21 +   8 +   0 +   3 +   0 +   6 +  12 +   0 +  18 +   1 +  15
  = 93
93 / 10 = 9 remainder 3
10 –  3 = 7

Thus, the check digit is 7, and the complete sequence is ISBN 978-0-306-40615-7.

In general, the ISBN-13 check digit is calculated as follows.

Let

${\displaystyle r={\big (}10-{\big (}x_{1}+3x_{2}+x_{3}+3x_{4}+\cdots +x_{11}+3x_{12}{\big )}\,{\bmod {\,}}10{\big )}.}$

Then

${\displaystyle x_{13}={\begin{cases}r&{\text{$ ; }}r<10\\0&{\text{ ; }}r=10.\end{cases}}}

This check system – similar to the UPC check digit formula – does not catch all errors of adjacent digit transposition. Specifically, if the difference between two adjacent digits is 5, the check digit will not catch their transposition. For instance, the above example allows this situation with the 6 followed by a 1. The correct order contributes 3×6+1×1 = 19 to the sum; while, if the digits are transposed (1 followed by a 6), the contribution of those two digits will be 3×1+1×6 = 9. However, 19 and 9 are congruent modulo 10, and so produce the same, final result: both ISBNs will have a check digit of 7. The ISBN-10 formula uses the prime modulus 11 which avoids this blind spot, but requires more than the digits 0–9 to express the check digit.

Additionally, if the sum of the 2nd, 4th, 6th, 8th, 10th, and 12th digits is tripled then added to the remaining digits (1st, 3rd, 5th, 7th, 9th, 11th, and 13th), the total will always be divisible by 10 (i.e., end in 0).

An ISBN-10 is converted to ISBN-13 by prepending "978" to the ISBN-10 and recalculating the final checksum digit using the ISBN-13 algorithm. The reverse process can also be performed, but not for numbers commencing with a prefix other than 978, which have no 10-digit equivalent.

Publishers and libraries have varied policies about the use of the ISBN check digit. Publishers sometimes fail to check the correspondence of a book title and its ISBN before publishing it; that failure causes book identification problems for libraries, booksellers, and readers.[50] For example, ISBN 0-590-76484-5 is shared by two books – Ninja gaiden®: a novel based on the best-selling game by Tecmo (1990) and Wacky laws (1997), both published by Scholastic.

Most libraries and booksellers display the book record for an invalid ISBN issued by the publisher. The Library of Congress catalogue contains books published with invalid ISBNs, which it usually tags with the phrase "Cancelled ISBN".[51] However, book-ordering systems such as Amazon.com will not search for a book if an invalid ISBN is entered to its search engine. OCLC often indexes by invalid ISBNs, if the book is indexed in that way by a member library.

Only the term "ISBN" should be used; the terms "eISBN" and "e-ISBN" have historically been sources of confusion and should be avoided. If a book exists in one or more digital (e-book) formats, each of those formats must have its own ISBN. In other words, each of the three separate EPUB, Amazon Kindle, and PDF formats of a particular book will have its own specific ISBN. They should not share the ISBN of the paper version, and there is no generic "eISBN" which encompasses all the e-book formats for a title.[52]