You are probably here because you want to validate routing and bank account numbers. There is good and bad news. Good news US bank routing numbers follow a very strict format. That makes it so you can easily validate it and be assured that before you send that data to the back-end it passes the format requirements. Bad News Bank account numbers are not so fortunate. There is no strict format unlike IBAN . Dang it! I discuss is more here . Routing Number You can get really fancy and do some really deep validation for the routing number. As you can see here there is a lot behind the format including what each set of digits represents (what kind of institution is the bank, what Federal Reserve district it is in, which state its in, etc). But we are going to keep it simple. There is a checksum. Thanks to BrainJar for presenting a simple solution. First off the number is always 9 digits. Next is the checksum algorithm. Here is our example rou