Account numbers: What they mean and how to find them

Account numbers are unique strings of characters that identify a specific financial account. This article explores their origins and precise meanings.

August 12, 2021

Jennifer Lowe
Jennifer Lowe

Jennifer is a writer at Plaid, covering lending and payroll. She believes in the potential of new financial services to enable greater financial access.


You've probably seen the long strings of numbers on the backs of credit cards or at the bottoms of paper checks. It’s clear that these numbers are involved with financial transactions, but what exactly do they signify? Where do they come from, and how are they used? 

This article will explore the history and precise meaning of account numbers. It will consider the difference between commonly used numbers like account-, routing-, and credit-card numbers—as well as how to find them. Finally, it will explore how account numbers are slowly giving way to more modern verification and security measures, ones less prone to errors and fraud.

What is an account number?

An account number is a unique string of numbers, letters, and other characters that identify a specific financial account. Almost all financial transactions make use of account numbers. Examples of account numbers include routing numbers and credit card numbers.

Bank accounts have existed since ancient Mesopotamia, but the account number is a more recent invention. Financial account numbers as we know them today are thought to have originated when computers began to be used for banking in the late 1960s. Since then, account- and routing numbers have helped to securely identify account holders and financial institutions, respectively.

In the United States, each bank assigns account numbers using its own methodology, kept private for security reasons. Most bank account numbers have between 8 and 12 digits, though they can range from 5 to 17. To protect account numbers from fraud, financial institutions may also use multi-factor identification, two-step verification, and other security measures.

Every financial transaction involves at least two account numbers: one that indicates where the money is coming from, and one that indicates where it is going.

Are account numbers unique?

Yes, the account number is like the fingerprint of the financial account. It does identify the account holder, but it's associated with other information as well. For example: physical address, phone number, email address, age of account, account balance.

A person may hold multiple accounts, each with its own unique account number. If a bank merges with or acquires another bank, account numbers may change. Banks must legally notify their account holders of any account changes.

What is a routing number?

In the same way that every financial account is assigned a unique account number, every financial institution is assigned a unique routing number. These financial institution routing numbers are known as RTNs (Routing Transit Numbers) or ABA (American Bankers Association) routing numbers. 

A routing number is a sequence of 9 digits. Small financial institutions typically have just one routing number, while larger multinational institutions can have several based on location. Routing numbers are issued only to federal- or state-chartered financial institutions eligible to maintain an account at a Federal Reserve Bank. If two banks merge, or one bank acquires another, the applicable routing numbers may change.

The first four digits of a routing number are called the Federal Reserve Processing Symbol.

  • The first two digits are typically a number between 01-12. These numbers refer to the head branch of the Federal Reserve District Office under which the bank falls. The Districts start with 01 in Boston and end with 12 in San Francisco and often encompass surrounding states. A range of 61-72 is assigned to non-bank payment processors and the number 80 to travelers’ checks.

  • The third digit identifies the Regional Federal Reserve Processing Center assigned to the bank within its district. For example, “1” would indicate bank check processing center 1.

  • The fourth digit indicates the bank’s location within a Federal Reserve District. A bank that is located in a Federal Reserve District is designated with a 0. Numbers 1-9 are used according to the state that the Federal Reserve District is in.

Digits five through eight comprise a unique identity code assigned to the bank by the American Bankers Association. 

The final digit of the routing number is called a “check digit.” Whereas the other digits refer to identifying information about the specific financial institution, the check digit is calculated using the other eight digits as a way to ensure authenticity and prevent fraud. (Thus “check” is used in the sense of “verify” rather than “paper check.”) 

Because no two financial institutions share all of the identifying information listed above, every routing number is unique.

What is the difference between routing numbers and account numbers?

Every bank-related financial transaction requires a routing number and an account number. The routing number identifies the specific bank. The account number (usually 10-12 digits) refers to the client’s account.

Routing numbers enable financial institutions to track where funds originated and where they’re going. Routing numbers are required for financial institutions to process transactions like direct deposits, check deposits, loan payments, and wire transfers.

Account numbers are unique to a specific financial account. Clients may hold multiple accounts, each with a different account number, while the routing number (i.e., the financial institution that holds the customer’s account) remains the same.

How do you find the account and routing numbers on a check?

The routing number is the string of nine numbers in the bottom left-hand corner of a paper check. The account number is the string of 5-17 digits in the bottom right-hand corner of a paper check. 

The routing number on a check specifies where the account was opened. The account number indicates a specific financial account and its associated account information.

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What is the difference between an ABA number and an ACH number?

ABA (American Banking Association) routing numbers are used for paper or check transfers and identifies the financial institution. ACH (Automated Clearing House) routing numbers identify the clearinghouse used for an ACH transfer. 

An ACH transfer is a way to send or receive money online. The clearinghouse is the designated financial intermediary that validates and finalizes transactions between a buyer and a seller. ABA routing numbers encompass all routing numbers, including ACH.

What are credit card numbers and what do they mean?

A credit card number is the 16-digit number that features prominently on the front or back of a credit card. It provides information about the card issuer and the client’s account information.

The first six digits are known as the Issuer Identification Number (IIN). 

The first digit is a Major Industry Identifier (MII). The MII shows to which industry the card issuer belongs. For example:

  • Credit cards issued by airlines have an MII of 1

  • The entertainment and travel industries have an MII of 3 (this includes Diners Club and American Express)

  • Visa cards have an MII of 4

  • MasterCard has an MII of 5

  • Discover has an MII of 6

  • MIIs 7-9 are reserved for the petroleum and healthcare industries, as well as national standards bodies

The next five digits identify the bank or network that issued the card. All cards issued by the same bank and program share their IIN. For example:

  • An IIN of 475050 indicates a Visa card (4) from JPMorgan Chase (75050)

  • An IIN of 526219 indicates a MasterCard (5) from Citibank (26219)

  • If a given credit-card program is very large, it can be assigned multiple IINs. For example, Bank of America Visa cards have several possible IINs.

Digits 7-15 provide a unique credit card account number. This number is different from any bank account numbers the cardholder may have. That’s because many people hold credit cards from financial institutions outside of their banks. Assigning a unique credit card number makes it easier to replace a lost or stolen credit card without affecting the cardholder’s bank account, and it can also help prevent fraud. 

The final digit in a credit card number is the “check digit,” which functions much the same way as it does in a routing number. It is automatically generated by applying the Luhn Formula to the credit card’s other fifteen digits, and it can be used to ensure the validity of the entire credit card number. 

American Express credit card numbers

The exception to the above rules is American Express, whose cards have only 15 digits. That’s because, while other card networks rely on banks to issue their cards, American Express issues its own cards. 

For American Express, because no issuing bank is specified within the credit card number, the card numbers exhibit the following structure:

  • A two-digit MII (37 or 34)

  • A two-digit number indicating the type of card and currency used (for example, “97” indicates an American Express credit card issued in the United States)

  • A seven-digit account number

  • A check digit

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Other numbers on a credit card: CVV and expiration date

The card verification value (CVV) and expiration dates are additional numbers on credit cards. They add an extra layer of security, especially for transactions when the card is not present (for example, when making an online purchase).

The CVV is a three-digit number found on the back of a credit card. It’s an important security measure because it verifies that the person attempting to use the account has the physical card in their possession (as opposed to merely the account number) when making online purchases. The CVV also reduces fraud risk, as merchants are legally prohibited from storing CVVs. (Note that, on American Express cards, CVVs have four digits and can be found on the front of the card.)

The expiration date is a four-digit number separated by a slash (example: 11/24). Credit cards wear out over time, and replacing the card regularly helps improve its physical reliability. In addition, the expiration date serves as a security measure when the card is not present during the purchase, such as during an online or over-the-phone transaction.

What is a debit card number, and how is it different from a credit card?

A debit card number is the 16-digit number that features prominently on the front or back of a debit card. It provides information about the card issuer and the client’s account.

Although every debit card is linked to a particular checking account, the debit card number is different from the checking account number. That makes it easier to replace a lost or stolen debit card without affecting the underlying checking account.  

Debit cardholders who share a joint checking account will nonetheless have different debit card numbers, even though the cards draw on the same funds. The reasoning is the same: if one cardholder loses their debit card, it can be replaced without compromising the other cards or the account itself.

A debit card number is generated in much the same way as a credit card number, with the Major Industry Identifier (first digit), Issuer Identification Number (digits 2-6), unique account number (digits 7-15), and check digit (digit 16). 

What is a SWIFT code?

A SWIFT (Society for Worldwide Interbank Financial Telecommunication) code is an alphanumeric string of 8 to 11 characters that SWIFT uses to identify financial institutions. Each institution has a unique code, which is also referred to as its bank identifier code (BIC) or ISO 9362 code.

SWIFT serves as the messaging system that enables a worldwide network of financial institutions to exchange funds. It is used internationally and for money transfers that cross national borders. SWIFT codes are often used when initiating wire transfers both from another country to the U.S. and from a U.S. bank to one in another country.

Each SWIFT code is generated using information about the particular financial institution using the following system:

  • The institution code (characters 1-4)

  • The country code (characters 5-6)

  • Location code (characters 7-8)

  • [Optional] Individual branch code (characters 9-11)


At one time, account numbers were the state of the art in securing and authenticating financial accounts. But increasingly, especially when compared with other technologies, they have become a source of errors, friction, and fraud.

New security measures, such as tokens, biometrics, two-factor authentication, and instant account verification, can supplement and strengthen account numbers. Meanwhile, financial institutions have begun to pursue other avenues for preventing and detecting fraud. For example, predictive analytics and machine learning can be used for anomaly detection. Such measures examine email account, IP, phone, physical address, and other information for out-of-pattern behavior.

Digital account opening has made it faster and easier for consumers to acquire new accounts. It’s also made it easier for financial fraud to occur. As the risk for financial identity theft increases, firms are challenged to invest in improved digital tools and processes to protect client account information and prevent financial fraud.

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