CUSIP check digit validator & calculator.

How the CUSIP check digit works

A CUSIP is 9 characters: a 6-character issuer number, a 2-character issue number, and 1 numeric check digit — used across the US and Canada to identify stocks, bonds, and other securities. Each of the first 8 characters is converted to a value (digits keep their value, letters A–Z become 10–35, and the rare symbols * @ # become 36, 37 and 38). Scanning those 8 values left to right, every second one (the 2nd, 4th, 6th and 8th character) is doubled; for every value, doubled or not, its tens digit and ones digit are added into a running sum. The check digit is whatever value would make that sum a multiple of 10: (10 − (sum mod 10)) mod 10.

This is a different checksum from an ISIN's: an ISIN expands every letter into two digits before running a standard Luhn pass right-to-left, while a CUSIP keeps each character as one value and doubles every other position scanning left-to-right. Worked by hand against real CUSIPs: Apple Inc. (037833100) sums to 30, giving check digit 0; Microsoft Corp (594918104) sums to 36, giving check digit 4; and Goldman Sachs Group (38141G104, which contains a letter) sums to 26, giving check digit 4 — all three match their real, publicly known CUSIPs.