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<title>David's Homepage: ISIN check digit algorithm</title>
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<h1 align="center">ISIN check digit algorithm</h1>
<p>The last digit of an ISIN is a check digit, calculated from
the other 9 digits which of the numerical portion of the ISIN,
using the so-called "double-add-double" algorithm. This
document will describe the algorithm, then apply it to an
example ISIN, then give an example function that will calculate
the check digit.</p>
<h2>Description of algorithm</h2>
<p>These are the steps of the calculation. Apply them to the
first nine digits of the ISIN to obtain the tenth.</p>
<ol>
<li>Starting with the right-most digit, replace each
alternate digit with the sum of the digits of its
double.</li>
<li>Take the sum of these digits. Take its last digit,
<i>d</i>.</li>
<li>The check digit is <i>9-d</i>.</li>
</ol>
<h2>Example: <code>GB0004005475</code> (HSBC)</h2>
<blockquote>
<pre>
<b> Original number: 0 0 0 4 0 0 5 4 7 [5]</b>
x2 alternate digits: 0 0 0 10 14
add digits: 0 0 0 1 5
<b>All digits after step 1: 0 0 0 4 0 0 1 4 5 [5]</b>
<b> sum of digits: 4 + 1 + 4 + 5 = 14</b>
last digit of sum: 4
<b> check digit: 9 - 4 = 5</b> -- correct answer!
</pre>
</blockquote>
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<p align="center"><a mailto="david@sheetmusic.org.uk">David Chan</a>
Tue Feb 27 16:16:48 2001</p>
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