Editing ISSN (section)

== Code format ==
An ISSN is an eight-digit code, divided by a hyphen into two four-digit numbers.<ref name=Whatis /> The last digit, which may be zero through nine or an X, is a [[check digit]], so the ISSN is uniquely represented by its first seven digits. Formally, the general form of the ISSN (also named "ISSN structure" or "ISSN syntax") can be expressed as follows:{{Ref RFC|3044}}
{{block indent |left=2 |text= {{Code|NNNN-NNNC}} }}
where <code>N</code> is in the set {''0,1,2,...,9''}, a decimal digit character, and <code>C</code> is in {''0,1,2,...,9,X''}; or by a [[Perl Compatible Regular Expressions]] (PCRE) [[regular expression]]:
{{block indent |left=2 |text= {{Code|lang=perl|1=^[0-9]{4}-[0-9]{3}[0-9X]$}}. }}

For example, the ISSN of the journal ''[[Hearing Research]]'', is 0378-5955, where the final 5 is the check digit, that is <code>C</code>=5. To calculate the check digit, the following algorithm may be used:

{{block indent |left=2 |text= Each of the first seven digits of the ISSN is multiplied by its position in the number, counting from the right, that is, 8, 7, 6, 5, 4, 3, and 2, respectively, and the resulting products are added:
<math display=block>\begin{align}
&0\cdot 8 + 3\cdot 7 + 7\cdot 6 + 8\cdot 5 + 5\cdot 4 + 9\cdot 3 + 5\cdot 2 \\
&= 0 + 21 + 42 + 40 + 20 + 27 + 10 \\
&= 160\;.
\end{align}</math>

The [[remainder]] of this sum [[modulo]] 11 is then calculated:
<math display=block>\frac{160}{11} = 14\mbox{ remainder }6=14+\frac{6}{11}</math>

If there is no remainder, the check digit is 0; otherwise the remainder is subtracted from 11. If the result is less than 10, it yields the check digit:
<math display=block>11 - 6 = 5\;.</math>
Thus, in this example, the check digit <code>C</code> is 5.

If the result is 10 (that is, if the remainder is 1), the check digit is an uppercase X (like a [[Roman numerals|Roman ten]]).
}} <!-- end long block indent -->

To confirm the check digit, calculate the sum of all eight digits of the ISSN multiplied by their position in the number, counting from the right. (If the check digit is X, add 10 to the sum.) The remainder of the sum modulo 11 must be 0. There is an online ISSN checker that can validate an ISSN, based on the above algorithm.<ref>{{cite web |url=https://journal-index.org/index.php/issnChecker|title=Online ISSN Validator |publisher=Advanced Science Index |access-date=16 Sep 2023 |archive-date=2 Aug 2023 |archive-url=https://web.archive.org/web/20230802120549/https://journal-index.org/index.php/issnChecker |url-status=live }}</ref>

=== In EANs ===

ISSNs can be encoded in [[International Article Number (EAN)|EAN-13 bar codes]] with a 977 "country code" (compare the 978 "[[bookland]]" country code for [[ISBN]]s), followed by the 7 main digits of the ISSN (the check digit is not included), followed by 2 publisher-defined digits, followed by the EAN check digit (in most cases, this will not match the ISSN check digit, since they are each calculated in a different way).<ref>{{cite book|url=https://www.issn.org/understanding-the-issn/issn-uses/identification-with-the-ean-13-barcode/|title=Identification with the GTIN 13 barcode|publisher=ISSN International Centre|archive-url=https://web.archive.org/web/20200629062603/https://www.issn.org/understanding-the-issn/issn-uses/identification-with-the-ean-13-barcode/|archive-date=June 29, 2020|url-status=live}}</ref>