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What is a CUSIP Number ​

CUSIP stands for Committee on Uniform Securities Identification Procedures and is pronounced Q-sip.

A CUSIP number identifies most financial securities in the United States and Canada, including: stocks of all registered U.S. and Canadian companies, commercial paper, and U.S. government and municipal bonds.

These numbers are used to help facilitate trades and settlements by providing a constant identifier to help distinguish the securities within a trade.

CUSIP numbers format ​

CUSIP numbers consist of nine characters (including letters and numbers).

  • The first six alphanumeric characters are known as the base or CUSIP-6, which identifies the issuer.
  • The seventh and eighth digits identify the type of security.
  • The ninth digit is a check digit that is automatically generated.

For instance,

Apple's common stock CUSIP is 037833100. Apple's company identifier is 037833 and the security identifiers are 10 for the common stock and the last 0 is the check digit.

And a CUSIP for one of Apple's outstanding bonds is 037833AK6. The company identifier is still 037833 and the security identifiers are AK for that specific bond and the last 6 is the check digit.

Some CUSIP numbers examples:

SecurityCUSIP
SPY ETF78462F103
VOO ETF922908363
QQQ ETF46090E103
Apple Stock037833100
Microsoft Stock594918104
Meta(Facebook) Stock30303M102

How to generate the check digit ​

The 9th digit is an automatically generated check digit using the "Modulus 10 Double Add Double" technique based on the Luhn algorithm.

  1. Begining at the rightmost character(the 8th character) and moving left, every second character is multiplied by two(including the rightmost character).

INFO

Letters are converted to numbers. The letter A will be 10, and the value of each subsequent letter will be the preceding letter’s value incremented by 1. In other words, A=10, B=11, C=12, ..., Z=35.

  1. Sum the values of the resulting digits (numbers greater than 10 become two seperate digits).
  2. The check digit is calculated by 10 - (s mod 10)) mod 10, where s is the sum from step 2. This is the smallest number (possibly zero) that must be added to s to make a multiple of 10.

For example, in the case of 037833AK6 , the payload is 037833AK and the calculation steps are as the following table.

cusip check digit algorithm

The swift language version validating the CUSIP check digit algorithm is as the following code.

swift
func isValidCUSIP( cusip:String) -> Bool {
    let cusipchars = Array("0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZ")
    let multiple = [1,2,1,2,1,2,1,2]
    
    if cusip.count != 9 {
        return false
    }
    let cusipArray = Array(cusip)
    var sum = 0
    for i in 0...7 {
        guard let index = cusipchars.firstIndex(of: cusipArray[i]) else {
            return false
        }
        let num = index * multiple[i]
        sum += (num%10 + num/10)
    }
    guard let check = Int(String(cusipArray[8])) else {
        return false
    }
    let calCheck = (10 - (sum % 10))%10 // or  9 - ((sum+9) % 10)
    return check == calCheck
}

References ​