Routing Numbers Explained

If you have been asked for a routing number for a business bank account, you have probably seen it as “RTN” or “ABA number”. This guide explains what the sources we could read say, on 2026-10-16. It is general information, not banking or legal advice.

What a routing number is

The Federal Reserve’s Micro Data Reference Manual describes the American Bankers Association (ABA) number as a nine-digit routing number, or routing transit number, used by banks to identify specific financial institutions, often for wires and ACH. It says the number consists of an eight-digit routing number and a check digit, that the routing number is used to clear checks and other means of payment, and that it embodies geographic information. It also notes the ABA number is assigned by Accuity to any depository institution applying for one, and in some cases by a Federal Reserve District Bank, and that no two concurrent entities may have the same ABA number.

Where to find yours

The Social Security Administration’s operations manual describes where to find routing transit numbers on account documents:

• On a check: the magnetic ink (MICR) line at the bottom of a check, where the manual’s exhibits show the routing transit number and the depositor account number. The check serial number may appear inside or outside the account number field; the manual says to ignore it.
• On an account statement or savings passbook: the routing transit number is not shown, so SSA tells staff to use other means of finding it.
• On a “payable through” check: the number on the check does not belong to the credit union or bank that holds the account, so SSA tells staff not to use it.

In practice, the safest source is your bank’s own website, your online banking, or a call or visit to your branch. If a payment form asks for a specific type of routing number, ask your bank which one to use.

Why businesses need it

ACH and other bank-payment forms ask for the routing number and the account number. Square’s pricing page, for example, says customers can pay an invoice by ACH with just their bank account and routing number. See ACH payments for small business for how those payments work and what they cost, and online invoices and payment links if you invoice clients.

A routing number is not a lookup tool

People often search for a “routing number lookup tool”. This site does not publish routing numbers or run a lookup, because we have not verified numbers against an official directory, and a wrong digit sends a payment to the wrong place. If you are choosing a bank, see do you need a business bank account? and our business bank account ranking.

What we did not verify

We did not read the Federal Reserve’s routing number directory, Nacha’s rules, or any bank’s own pages on routing numbers. We did not verify the different numbers a bank may use for ACH, wires or checks. The SSA page is guidance for its own staff, written in 2018, and we use it only for the description of where numbers appear on documents.