ABA routing numbers have been around for over 100 years, but unless you've worked in banking, you probably haven't paid much attention to them.

Also known as bank routing numbers, these codes are part of a system that helps banks identify each other and ensure that your checks and electronic transactions are processed in a speedy and cost-effective manner.

Like the home address of a good friend, you don't need to memorize your bank's ABA routing number, but you do need to have it handy for certain occasions.

What is a bank ABA routing number?

ABA routing numbers are nine-digit codes that identify financial institutions. These numbers are part of a standardized system designed by the American Bankers Association (ABA) in 1910 to make sure that check transactions went through to the right banks. Today, only 3% of payments are made with checks, but ABA routing numbers are still widely used to facilitate both check and electronic transactions.

Every ABA routing number is formatted as follows:

• Digits 1-4: The routing number for the Federal Reserve district. The first two digits always start with between 00 and 12.

• Digits 5-8: The identity of the financial institution.

• Digit 9: The check digit, which verifies the authenticity of the routing number.

Do I need to know my bank's ABA routing number?

You don't need to memorize your bank's ABA routing number, but you will need to look it up once in a while. Without both your account number and your bank's ABA routing number, you can't set up these essential transactions:

• Wire transfers

• IRS tax refunds

• Loan applications

• Online bill payments

• Automatic withdrawals

• Transferring funds

• Re-ordering checks

• Direct deposits from your employer or Social Security

How to look up your bank’s ABA routing number

To find your bank's ABA routing number, you can log in to your bank account through your browser or banking app, then select the account and look for the information.

ABA routing numbers, which are sometimes called check routing numbers, are also printed on the checks issued to you by your bank, deposit slips, and bank statements. On a check, the routing number is the first series of numbers you see on the bottom, left-hand corner of the front side. The numbers that follow it are your account number and check number, respectively.

Additionally, you can look up the number by using the ABA's Routing Number Lookup tool — but you might end up finding the wrong number. That's because large banks have multiple routing numbers. For example, Bank of America uses a unique ABA routing number for each state, and the correct number depends on the state where the account was issued. So it's best to call your bank and confirm, or look at your account-specific information.

Read more: These are the 20 biggest banks in the US

What happens if I use the wrong ABA routing number?

If you use the wrong ABA routing number, you'll have problems with your transaction. The transaction could be delayed, rejected, or worse: It could go through to the wrong account.

For state and federal tax refunds, your bank will likely reject the deposit, and you'll receive the refund by check. If so, the check will be sent to the address on your tax return and addressed to the people whose names are on the return. In other scenarios, you'll need to be more proactive.

Before the transaction goes through

If the transaction is still pending, contact the issuing institution right away. Here's what you can do:

• Banks: For scheduled transfers, you may need to notify your bank at least three days before the scheduled date. Your bank may require a written notice in addition to your oral notice.

• Merchants: If you've authorized a merchant to credit or debit your account, contact them to correct the information. You may need to revoke authorization for a transaction in writing and ask if you can be issued a cancellation number.

• The IRS: You can't change your routing number after you file your tax return, but you can call the IRS at 800-829-1040 to request a check.

After the transaction goes through

If the money goes to the wrong account, you'll have to work with the agency that received the funds to find a solution. They may be able to move the funds to the correct account or return them to the issuer, but they might require you to submit a payment reversal request.

Unfortunately, the company that received the money could also refuse to return it, in which case you might have to take legal action to recover the funds.

FAQs

Is ABA the same as a routing number?

ABA is not the same as a routing number. ABA is an abbreviation for American Bankers Association, which is the organization that developed the ABA routing number system.

What is an example of an ABA code?

An ABA code, or routing number, is a nine-digit code with the first two digits ranging from 00 to 12. So an example of an ABA code is 123456789.

Are all ABA numbers 9 digits?

Yes, all ABA routing numbers are nine digits long.

Are ABA routing numbers the same as RTNs?

ABA routing numbers are the same as routing transit numbers (RTNs). You may also see them referred to as bank routing transit numbers.