From Your Bank
Go to your bank's website and look for the page for routing number information. Though the steps vary by financial institution, you will typically look or be asked for the state where you opened or maintained the account. The number will be listed by the location of your account and whether the transaction involves your checking or savings account or a wire transfer. If you bank online, log in to your account information. Depending on the bank, you'll see the number with your other account information or on your online statement.
From the ABA
The ABA's Routing Number Lookup page allows you to find your routing number. Click "Agree" at the bottom of the user agreement. On the search page, you will enter your bank's name and the state and zip code of the branch of your bank. Depending on your entries, you'll see several listings. Find the routing number on the listing for the address of your branch.