ISIN Format

ISIN Format

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.

    1. 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 ISIN.org and try it yourself as you will be amazed at how reliable the calculation is.

The International Securities Identification Numbers Organisation is an information Service to the financial industry and to individual Investors.