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An introduction to ABA numbers

When you make a direct deposit or send a wire transfer, how does your money move from one bank account to another? One identifying factor is your account’s ABA number, or bank router number. So, what is a bank ABA number, and how does it work?

What is a bank ABA number?

A bank ABA number consists of nine digits. Also called a bank routing number or routing transit number (RTN), it’s used to identify US banks and transfer money from one account to another. Every bank in the country has its own ABA routing number, which identifies the bank’s location when processing payments – much like a digital address.

You’ll often find your ABA number on checks, printed using a special magnetic ink to make them computer readable. Even when they’re not printed magnetically, a MICR font makes it easier for machines to automatically recognize the numbers. If you’re ever deposited a check into your bank account by taking a smartphone photo, this is how it works.

History of ABA routing numbers

ABA routing numbers have been used for over 100 years in the United States. They were created in 1910 by the American Bankers Association (ABA) as paper check processing became more common. Routing numbers made it easier to identify which banks were issuing the payments.

While most payments are electronic now, ABA routing numbers are still useful for directing bank transfers. The system has evolved to reflect Federal Reserve procedures and facilitate online payments.

How do ABA numbers work?

Together, your bank account number and ABA number provide all the identifying details needed to process payments. For account holders, no further action is needed once you’ve provided these numbers to a payment processor. You might receive a new ABA number if your bank undergoes a merger or you sign up for new services, but in many cases the routing number is automatically transferred.

What do the numbers themselves mean? Here’s how they are assigned:

  • Numbers 1 to 4: The first four digits of any ABA number were originally designed to represent a bank’s geographical location, assigned by the Federal Reserve Routing System. However, as banks have moved around over the years due to acquisitions and mergers, these numbers don’t always correlate with its current location.

  • Numbers 5 to 6: These correspond to the Federal Reserve bank that electronic and wire transfers will be routed through.

  • Number 7: The seventh digit corresponds with the bank’s Federal Reserve check processing center.

  • Number 8: This digit lets you know which Federal Reserve district the bank is located in.

  • Number 9: The final digit in an ABA number is a “checksum,” which is a mathematical sum of the first eight numbers. The final result should be equal to this checksum or else the transaction will be rerouted.

How to find an ABA number

When setting up automatic payment services or wire transfers, you’ll be asked for your ABA number. The easiest way to locate this is to look for a 9-digit number on a physical check or deposit slip. The ABA number on checks is typically featured in the bottom left-hand corner. 

If you don’t use paper checks, you can also log into your online bank account to find the ABA number. Perform a quick search for Automated Clearing House (ACH) and it should pop up there or under your bank’s direct deposit forms.

One thing to note is that larger banks often use several ABA routing numbers. You’ll need the one that applies to your account. This usually depends on where the account was opened and whether the bank has gone through mergers since then. There might also be separate ABA numbers for direct deposit and wire transfers. When in doubt, give customer service a call to ask.

ABA numbers and international transfers

Nine-digit routing numbers are only used within the US, while most other countries use an IBAN instead. If you’re abroad and wish to transfer funds into a US bank account, you’ll need to provide the ABA number. For international payments you’ll usually need the account’s BIC/SWIFT code as well, which can be found on the bank’s website or calling customer service.

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