Convert 9-digit CUSIP codes into ISIN codes

Question

How do I convert 9-digit CUSIP codes into ISIN codes, preferably in Excel?

Answer 1

The CUSIP is entirely contained in the ISIN. The ISIN is then prefixed by 2 letters (in this case, either "US" or "CA" as CUSIP is a North American identifier) and a digit at the end.

To determine the digit at the end:

ISIN US0378331005, expanded from CUSIP 037833100. The main body of the ISIN is the original CUSIP, assigned in the 1970s. The country code "US" has been added on the front, and an additional check digit at the end. The country code indicates the country of issue. The check digit is calculated in this way.

Convert any letters to numbers:

U = 30, S = 28. US037833100 -> 3028037833100.

Collect odd and even characters:

3028037833100 = (3, 2, 0, 7, 3, 1, 0), (0, 8, 3, 8, 3, 0)

Multiply the group containing the rightmost character (which is the FIRST group) by 2:

(6, 4, 0, 14, 6, 2, 0)

Add up the individual digits:

(6 + 4 + 0 + (1 + 4) + 6 + 2 + 0) + (0 + 8 + 3 + 8 + 3 + 0) = 45

Take the 10s modulus of the sum:

45 mod 10 = 5

Subtract from 10:

10 - 5 = 5

Take the 10s modulus of the result (this final step is important in the instance where the modulus of the sum is 0, as the resulting check digit would be 10).

5 mod 10 = 5

So the ISIN check digit is five.

_{Source: http://en.wikipedia.org/wiki/International_Securities_Identification_Number}