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CUSIP / ISIN

General

Committee on Uniform Securities Identification Procedures Number / International Securities Identification Number

CUSIP is a 9-character US/Canada securities identifier and ISIN is its 12-character global superset, both validated with modified Luhn checksum algorithms.

Definition

CUSIP (Committee on Uniform Securities Identification Procedures) is a 9-character alphanumeric code that uniquely identifies a North American security — stocks, bonds, and other financial instruments — for clearing, settlement, and recordkeeping. ISIN (International Securities Identification Number) is a 12-character global identifier that extends this concept worldwide: it prefixes a 2-letter country code onto a national security code (often the CUSIP itself, for US and Canadian securities) and appends its own check digit.

Together, the two systems let financial institutions unambiguously reference the same security across domestic (CUSIP) and cross-border (ISIN) transactions.

Formula

Both CUSIP and ISIN use a modified Luhn algorithm, but applied over different lengths and alphabets:

CUSIP (9th character is the check digit, derived from the first 8):

  1. Convert each character to a value: digits keep their value; letters take 10 + their alphabet position (A=10, B=11, ...); *, @, # map to fixed values 36, 37, 38.
  2. Double the value of characters in even positions (2nd, 4th, 6th, 8th).
  3. If a doubled value exceeds 9, sum its digits (equivalent to subtracting 9).
  4. Sum all resulting values; the check digit is (10 − (sum mod 10)) mod 10.

ISIN (12th character is the check digit, derived from the first 11, which already include the CUSIP or local code and country prefix):

  1. Convert all letters to their numeric equivalents (A=10 ... Z=35), producing a longer numeric string.
  2. Apply the standard Luhn algorithm to that numeric string, doubling every second digit from the right.
  3. The check digit is whatever makes the total sum a multiple of 10.

Worked Example

A structurally valid CUSIP example: 037833100 (fictional, illustrative format). Its corresponding illustrative ISIN, with the US country prefix and recomputed check digit: US0378331005.

Key Things to Know

  • CUSIP is 9 characters, ISIN is 12: an ISIN is typically a 2-letter country code plus a 9-character national code (often the CUSIP) plus its own 1-digit checksum.
  • Both use modified Luhn checks: the CUSIP/ISIN Validator verifies both checksums independently — a valid CUSIP inside an ISIN doesn't guarantee the ISIN's own check digit is also correct, and vice versa.
  • Different issuing bodies: CUSIP Global Services assigns CUSIPs for the American Bankers Association; national numbering agencies (often the same bureau, for US/Canada) assign ISINs per country.
  • Not the same as a ticker symbol: tickers identify a security for exchange trading and can change; CUSIP/ISIN identify the security itself for settlement and stay fixed.
  • Comparable in role to IBAN for accounts: much like an IBAN standardizes bank account identification internationally, ISIN standardizes security identification internationally.
  • Used across the trade lifecycle: from order execution through clearing and custody, CUSIP/ISIN travel with a security across every institution that touches the trade.

Related Terms

IBANInternational Bank Account NumberSWIFT / BICSociety for Worldwide Interbank Financial Telecommunication / Bank Identifier Code

Frequently Asked Questions

A CUSIP's 9th character is a check digit computed with a modified Luhn algorithm: each of the first 8 characters is converted to a numeric value (letters become 10 + their alphabet position), digits in even positions are doubled, the digits of each result are summed, and the check digit is whatever makes the grand total a multiple of 10. You can test a CUSIP or ISIN against these rules with the [CUSIP/ISIN Validator](/validators/cusip-isin-validator/).
An ISIN wraps a national security identifier (often a CUSIP for US/Canada securities) with a 2-letter country code prefix, then applies its own modified Luhn checksum across the full 11 preceding characters to produce the 12th and final digit. So a CUSIP-based ISIN effectively has two separate check digits embedded in it — the CUSIP's own 9th character, and the ISIN's 12th character covering the whole string.
US and Canadian securities typically have a CUSIP, and most of those CUSIPs are also embedded directly inside a corresponding ISIN for international trading and settlement. Securities issued outside North America generally have only an ISIN, built from a different national numbering agency's local code rather than a CUSIP.
No. A ticker symbol (like AAPL) identifies a security for trading on a specific exchange and can change if a company relists or rebrands, while a CUSIP is a fixed 9-character identifier tied to the specific security itself, used for clearing, settlement, and back-office recordkeeping rather than trading floor display.
CUSIP numbers are assigned by CUSIP Global Services, operated on behalf of the American Bankers Association, while ISINs are assigned by the national numbering agency in each security's country of registration — the CUSIP Service Bureau performs this role for US and Canadian ISINs. Both systems are coordinated internationally to avoid duplicate assignment.