The Effect of Corporate Actions on ISINs

We think of ISINs as immutable identifiers tied to a unique security, and that understanding is substantially correct. However, like all rules, there are exceptions – in this case, related to corporate actions. Corporate actions are status changes to a company or its securities. Sometimes, these status changes require that ISINs be amended or deleted. Let's take a comprehensive look at the effect of corporate actions on ISINs.

    A company changes the country in which it is headquartered: A new ISIN is required only if the company replaces an old security with a new one.

  1. Merger and acquisition: Old ISINs for stock become inactive and are replaced by securities with a new ISIN. Bonds only need new ISINs if old ones are exchanged for new ones.
  2. Redeemable or convertible debt securities are redeemed: The bond ISIN's become inactive.
    Bankruptcy and/or liquidation: Once the company is delisted, the ISIN becomes inactive.
    New stock issued or change to stock par value: Only need a new ISIN if new stock has different rights or is meant to replace old stock.
  3. Company changes its name: Replaced shares get a new ISIN, replaced debt securities receive an amended ISIN.
  4. Stock splits: A new ISIN is required when old certificates are exchanged for new ones. If the old certificated are simply stamped, the ISIN is retained.
  5. Reverse splits: A new ISIN is required when old certificates are exchanged for new ones.
    Coupon stripping into principal and interest strips: The strips should have the same prefix as the underlying instruments. If the stripping is performed by a trust or special purpose vehicle, then the ISIN should bear the prefix of the country in which the stripper is domiciled.

All of these actions are handled by issuers and their transfer agents, who must alert shareholders to any such actions. In sum, a new ISIN is required when an old security is replaced by a new one; otherwise, an ISIN need not change. ISINs are retired when a company or security ceases to exist, through merger, acquisition, bankruptcy, liquidation, or name change.

How ISIN Check Digits are Calculated

    The structure of an ISIN is composed of three components:

  1. A two-letter country code
  2. A nine-character alphanumeric security identifier
  3. A single check digit.

The check digit is used to help ensure the authenticity of the ISIN. But how is it calculated?
The basic idea is to establish a numerical value for every capital letter in the alphabet. Here is a table with the official conversions:

Computer geeks will recognize these numbers: they are equal to the ASCII code for each capital letter minus 55. For instance, the ASCII code for 'A' is 65, 'B' is 66, and so forth.

    The algorithm for calculating the ISIN check digit from is the following:

  1. Convert any alphabetic letters to their numeric equivalents using the above table.
    Beginning with the least significant digit (on the right), multiply every other digit by 2.
    Add up the resulting digits, calling the result SUM.
  2. Find the smallest number ending with a zero that is greater than or equal to SUM, and call it VALUE.
  3. Subtract SUM from VALUE, giving the check digit.

The technical term for the check digit is the ten's complement of the sum modulo 10. In English, this means the resulting sum, including the check digit, is a multiple of 10.

      Here is an example using Apple Inc. Its ISIN is US0378331005. As you can see, the check digit is five. Let's step through the process:

    1. U =30 and S=28, so the string (minus the check digit) becomes: 3028037833100
    2. Multiply every other digit by two:
      (3, 2, 0, 7, 3, 1, 0) becomes (6, 4, 0, 14, 6, 2, 0)
    3. Calculate SUM by adding up all the digits:
      SUM = (6 + 4 + 0 + (1 + 4) + 6 + 2 + 0) + (0 + 8 + 3 + 8 + 3 + 0) = 45
    4. Find VALUE, which is the smallest number greater than or equal to 45 that end with a zero. That's easy, VALUE = 50.
    5. Subtract SUM from VALUE giving the check digit:
      VALUE – SUM = 50 – 45 = 5 = check digit.

And indeed, the check digit of Apple Inc. is five.

We suggest you look up an ISIN here on and try it yourself – you will be amazed at how reliable the calculation is.

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