An ABA (or "routing") number is a code that identifies your bank. This code helps other banks transfer money to and from your account; it is similar to an address which tells everybody where to find your account.
ABA numbers are also known as routing transit numbers (RTNs). “ABA” is used because the American Bankers Association (ABA) assigns the numbers to banks.
Finding and Using ABA Numbers
How do you know what your ABA number is? You can get this information in several ways, but the easiest way is to look at your checkbook – your ABA number is printed on each check. It is the nine digit number in the bottom left-hand corner (it’s the farthest down and farthest to the left). In the image above, the ABA number is highlighted in yellow (view larger).
You can also contact your bank and ask which ABA number to use. Some banks even provide this information when you’re logged in to your account; search for direct deposit forms or Automated Clearing House (ACH) information.
Your bank may have several ABA numbers, but you should use the one specific to your account, and that’s the one you’ll find printed on your checks. ABA numbers may differ depending on where you opened your account, and bank mergers can result in multiple codes for the same bank.
How ABA Numbers Work
For the most part, all you need to do is copy your ABA number and provide it to whoever is asking for it – you don’t need to know anything more about these numbers. However, if you’re curious, there is a system behind the composition of ABA numbers. You can learn what each of the nine digits is for in Deciphering Bank Routing Numbers.
ABA numbers are used whenever you transfer money between banks, which is quite often. This happens when you write a check to somebody (their bank uses the ABA number to find your bank), or when you receive your wages through direct deposit. ABA numbers are also important any time you want to set up electronic payments for your mortgage, utilities, or insurance (using ACH debit, for example). It's hard to imagine how the banking system would function without these numbers.
In addition to your ABA number, you also need to provide your account number when setting up a link between your bank account and another organization. The ABA number only identifies your bank; it does not identify your account at that bank.