The ABA routing number , also known as the routing transit number, is a nine digit number associated with a financial institution. If you look at a personal check, it’s one of the three numbers at the bottom of the check (the others are the check number and your account number). Ever bank in the United States as an associated ABA routing number it tells various financial systems where to transfer money from and to.
There are cases where you start with an ABA routing number and need to find out the bank it belongs to. Perhaps you just want to double check that the correct numbers are printed on your checks. Maybe you received a check and want to confirm it’s legitimate, checking the ABA routing number is a good first step.
Whatever the case, the easiest place to do a reverse ABA routing lookup is through this free tool  from Lyons Commercial Data.
It is by far the easiest and quickest ABA routing number reverse lookup I could find.