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DIVEC / Business-ISIN-0.01 / isin-sum.html 
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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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