What Is an ABA Number?

how ABA numbers work

The Balance

An ABA number, also known as a bank routing number, is a nine-digit code that identifies banks in the U.S. That number makes it possible for banks to transfer money to and from your accounts for transactions like wire transfers, direct deposit, and automatic bill payments.

How To Find and Use ABA Numbers

You can get your account’s ABA number from several sources. If you have a checkbook handy, the easiest solution is to locate the numbers at the bottom of one of your checks.

On Paper Checks

An ABA number is printed on every check. On personal checks, it is usually the nine-digit number in the bottom left-hand corner. The number might appear somewhere else on computer-generated checks (like online bill payment checks or business checks). You can also find your ABA number on deposit slips, typically in the same location.

Contact Your Bank

Some banks provide this information online, although you might need to log into your account to find the right number. Search your bank’s website for direct deposit forms or Automated Clearing House (ACH) information. Alternatively, call customer service and ask.

Use the Correct Number

Your bank may operate under several ABA numbers, so it’s essential to identify the one specific to your account. ABA numbers may differ depending on where you opened your account, and bank mergers can result in multiple codes for the same bank. Some banks also use separate ABA numbers for wire transfers vs. direct deposit or ACH transactions.

Even if you know the correct number for ordering checks, you might need to use a different number for wire transfers or electronic bill payments. When in doubt, ask a customer service representative at your bank which number to use.

How ABA Numbers Work

In most cases, all you need to do is provide your ABA number, along with your account number, to whoever is requesting it. Banks, billers, and your employer (or whoever else is setting up automatic transfers) handle the logistics after that.

If your bank fails or merges, you may receive new ABA numbers—but you don’t necessarily have to start using them right away.

Ask your bank if you can continue using old numbers until you order new checks or sign up for new services. In some cases, you can continue using old routing numbers indefinitely.

ABA numbers follow an intricate system:

Behind the Name

An ABA number is like an address that tells financial institutions where to find your account. As a result, ABA numbers may also be called routing transit numbers (RTNs) or check routing numbers.


Routing numbers are typically printed on checks using magnetic ink, which allows special machines to read the code more easily. Whether or not magnetic ink is present, printers usually use MICR font, making it easy for computers to visually recognize the numbers. That’s helpful when you deposit a check by snapping a photo with your mobile device, for example.

The first four digits are made up of the Federal Reserve Routing Symbol. The first two digits have different meanings based on their series. The ABA outlines each series on its website.

The next four digits are the ABA institution identifying digits.

The ninth digit provides a checksum or check digit. This is a complicated mathematical expression using the first eight digits. If the end result does not equal the checksum number, the transaction gets flagged and rerouted for manual processing.

History of the ABA Number

In 1910, the American Bankers Association (ABA) created ABA numbers as a way to assign unique identifiers to each payment-issuing bank. Doing so reduced confusion for processing paper checks, which were a common form of payment at the time. Over time, ABA numbers have evolved to include the Federal Reserve’s payment processes and facilitate modern electronic payments.

How ABA Numbers Help Process Checks

Since the 1960s, the ABA number has played an instrumental part in significantly speeding up check processing. Additionally, with the passage of the Check 21 Act in 2003, physical checks that needed to travel by airplane and truck to reach banks could now be submitted and cleared electronically. As a result, funds clear much more quickly, and consumers no longer have the luxury of "playing the float" or issuing a check a few days before they actually have the funds in their account.

Key Takeaways

  • An ABA number identifies financial institutions so they can conduct transactions such as wire transfers and direct deposits.
  • ABA numbers have nine digits, and you can usually find them in the bottom left-hand corner of a personal check.
  • An ABA number is also referred to as a bank's routing number.
  • The American Bankers Association (ABA) created ABA numbers in 1910.

Frequently Asked Questions (FAQs)

How can you tell the difference between a routing number and an account number?

The routing number is usually the first, bottom-left number on your check and is followed by the account number. The routing number is always nine digits long, while your account number may be between 10 and 12 digits.

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

IBAN numbers, or international bank account numbers, are used for international bank routing throughout much of Europe. ABA numbers are specific only to the U.S. The U.S. does not use IBAN numbers for international banking. For international transfers, the U.S. uses SWIFT codes.

Article Sources

  1. U.S. Bank. "Find Your U.S. Bank Checking Account Routing Number."

  2. Bank of America. "Routing Number FAQs."

  3. Xerox. "Generic MICR Fundamentals Guide," Page 1-5.

  4. American Bankers Association. "Routing Number Policy & Procedures."

  5. FRASER, Federal Reserve Bank of St. Louis. "8203. Addition of the Check Digit in the Routing Number Field on Checks."

  6. American Bankers Association. "ABA Routing Number."

  7. Board of Governors of the Federal Reserve System. "Regulation CC (Availability of Funds and Collection of Checks): Frequently Asked Questions About Check 21."

  8. Board of Governors of the Federal Reserve System. "The Federal Reserve in the Payments Mechanism," Page 11.

  9. Bank of America. "Wire Transfers FAQs."