Routing Number

A routing number is the identifier for a bank in the United States payment system. Regulation CC, the federal check-collection rule, defines it at 12 CFR 229.2(dd) in three limbs: the number printed on the face of a check, in either fractional or nine-digit form; the number in a bank's endorsement, in either form; and, for the subparts governing check collection and substitute checks, the bank-identification number carried in an electronic check.

Three names are in circulation for the same thing, and none of them is wrong. The official register calls it a routing number; "routing transit number" and "ABA routing number" are both in common use, the second because the numbers are assigned by an agent of the American Bankers Association rather than by a government body; and bank forms often shorten it to "ABA number". A reader who has only met one of these should recognize the other three.

The number is one half of a pair, and the other half is worth naming precisely because the two get confused. The routing number identifies the institution; the account number identifies the particular account inside it. Federal regulation says almost nothing about the second: Regulation E's entire definitional treatment of the phrase, in the list of what a periodic statement must show, is "Account number. The number of the account" (12 CFR 1005.9(b)(2)). That is not an oversight so much as a reflection of what an account number is. It is an internal identifier each institution issues under its own conventions, with no national format and no register, which is exactly why it cannot do the routing job and why a payer generally asks for both numbers together.

Who assigns them, in Regulation CC's own commentary. The official commentary to 229.2(dd) begins: "Each bank is assigned a routing number by an agent of the American Bankers Association." The same passage sets out the two printed forms: a paying bank is identified by both the fractional form, "which normally appears in the upper right hand corner of the check", and the nine-digit form, which "generally is printed in magnetic ink near the bottom of the check (the MICR line; see ANS X9.13)". For an electronic check, the commentary adds, the routing number is contained in the image of the check in both forms and in the electronic information related to the check in nine-digit form. So one bank identifier has two printed shapes and one electronic one, and paper checks are generally handled by collecting banks on the basis of the nine-digit version.

The digits are not arbitrary. Appendix A to Regulation CC explains the structure: "The first four digits of the nine-digit routing number (and the denominator of the fractional routing number) form the 'Federal Reserve routing symbol,' and the first two digits of the routing number identify the Federal Reserve District in which the bank is located." The appendix gives its own examples: 01 opens the routing number of a bank in the First Federal Reserve District, which is Boston, and 12 opens one in the Twelfth District, which is San Francisco. It adds a second convention: "Adding 2 to the first digit denotes a thrift institution. Thus, 21 identifies a thrift in the First District, and 32 denotes a thrift in the Twelfth District." The leading digits of a routing number therefore carry geography and institution type, which is why they cluster by region.

The number on a check is not always the bank that pays it. The same commentary states that "when a check is payable by one bank but payable through another bank, the routing number appearing on the check is that of the payable-through bank, not the payor bank." A routing number is an instruction about where to send the item, not a statement about which institution holds the money, and those can differ. This is the structural reason a reader should not treat a single routing number as an institution's identity. It also explains why the practical advice is to use the number the institution gives for the specific kind of transfer being set up, rather than reusing one number found on a check for every purpose.

Routing numbers are retired, and a retired one stops working. The rule sits in the Federal Reserve Banks' own Operating Circular No. 3, which governs their check service. Under paragraph 3.1, a sender "should not send to us any item, and we do not undertake to handle an item as a cash item, if ... the item includes in the MICR line, a routing number that has been retired by the American Bankers Association's Registrar of Routing Numbers" (Operating Circular No. 3, effective January 5, 2026). Note the precise scope: the Reserve Banks decline to handle such an item through ordinary cash-item collection, which is the channel almost every consumer check travels. The Board of Governors described the same provision when it took effect, recording that "in 2015 the Federal Reserve Banks revised Operating Circular 3 ... to provide that the Federal Reserve Banks will not handle checks drawn on routing numbers that have been retired" (82 FR 27552, June 15, 2017). Retirement usually follows a merger or a consolidation of charters. The consequence for an account holder is concrete: an old checkbook, or an autopay instruction set up years earlier, can carry a number that no longer routes anywhere.

Where to find the offices behind a number, if it matters: the commentary points to the American Bankers Association Key to Routing Numbers, "published by an agent of the American Bankers Association, which lists a city and state address for each routing number."

A payer is given two numbers, the routing number and the account number. The payment system uses the first to deliver the item to the right institution and the second for that institution to post it to the right account. On a paper check both are printed in magnetic ink along the bottom, with the routing number also appearing in fractional form near the top right; for an electronic payment they are keyed into a form or a payroll portal.

A hypothetical illustration of what a retired routing number does, using invented amounts. Ines's credit union merged into a larger one two years ago. She still has half a checkbook from the old institution and writes a $340 check from it to a contractor, on the reasoning that the account is the same account.

The contractor deposits it and the check comes back. The routing number printed on the old stock has been retired, and the Federal Reserve Banks do not undertake to handle an item bearing a retired routing number as a cash item, so the check cannot be collected in the ordinary way. Ines has to pay the contractor again from current check stock or by another method, and depending on the contractor's own agreement with her, she may face a returned-payment charge for a check that was never short of money.

The reading exercise is worth doing once. Take the nine-digit number along the bottom of a check and look at the first two digits: they name the Federal Reserve District of the institution, and Regulation CC's appendix gives its own two examples: a number beginning 01 belongs to an institution in the First District (Boston) and one beginning 12 to the Twelfth (San Francisco). Add two to the first digit and you have the thrift-institution range for the same district. None of that is needed to make a payment, and all of it explains why the numbers you see are not random.

Pros

• It is a public identifier with a national register, so a payer can be given it without giving away anything private.
• The structure is documented, which makes an obviously malformed number easy to spot before a payment is set up.
• Regulation CC recognizes the electronic form as well as the two printed ones, so the same identifier works whether the item travels as paper or as an image.

Cons

• It identifies a bank and nothing more, so a correct routing number with a wrong account number still sends the money to the wrong place.
• The number printed on a check can be a payable-through bank's rather than the bank that holds the account, so it is not a reliable statement of who your bank is.
• Numbers are retired after mergers, and old check stock or an old autopay instruction can carry one that no longer routes.
• An institution may use different numbers for different kinds of transfer, so the number that works for one purpose is not guaranteed to be the right one for another. Ask the institution which number applies to the transfer being set up.
• Because it is public, it is also the number a fraudster needs least, which is why the private half of the pair deserves the care people often give to the public one.