A routing transit number (RTN) is a nine digit bank code, used in the United States, which appears on the bottom of negotiable instruments such as checks identifying the financial institution on which it was drawn. This code was designed to facilitate the sorting, bundling, and shipment of paper checks back to the drawer's (check writer's) account.

The RTN is also used by Federal Reserve Banks to process Fedwire funds transfers, and by the Automated Clearing House to process direct deposits, bill payments, and other such automated transfers.

The RTN number is derived from the bank's transit number originated by the American Bankers Association, which designed it in 1910

ABA number management

Since 1911, the American Bankers Association has assigned transit numbers through a series of registrars, currently Accuity. The company is responsible for assigning new ABA numbers. Accuity publishes the ABA Number Directory in the American Bankers Association Key to Routing Numbers semi-annually.

There are approximately 26,895 active routing and transit numbers currently in use. Every financial institution in the United States has at least one of these. Multiple RTNs may result from mergers; the Fed requires banks to phase these out (to a single surviving RTN per state, or fewer) within about 18 months. Once support for such "old paper" is dropped, customer's checks will "bounce" and be returned unpaid to the merchant or originator.

ABA numbers are only for use in domestic transactions within the United States and are of two types, one for funds being debited or credited to or from accounts and one that is used for wire transfers. They are different and usually the ABA number on a check book which is usually the middle set of nine numbers printed at the bottom of the check is the former. Domestic transfers that use the debit/credit routing number will usually be returned to the sending bank. Incoming international wire transfers use a different code system call SWIFT-BIC, BIC code, SWIFT ID or SWIFT code more of which can be read about under ISO 9362. There are a number of overlapping issues between these codes and complicating the matter is the fact that European Banks use an IBAN code.

The IBAN was originally developed to facilitate payments within the European Union but the format is flexible enough to be applied globally. It consists of a ISO 3166-1 alpha-2 country code, followed by two check digits that are calculated using a mod-97 technique, and Basic Bank Account Number (BBAN) with up to thirty alphanumeric characters. The BBAN includes the domestic bank account number and potentially routing information. The national banking communities decide individually on a fixed length for all BBAN in their country.

Routing number format

The ABA transit number appears in two forms on a standard check – the fraction form and the MICR (magnetic ink character recognition) form. Both forms give essentially the same information, though there are slight differences.

The MICR forms are the main form – it is printed in magnetic ink, and is machine-readable; it appears at the bottom left of a check, and consists of nine digits.

The fraction form was used for manual processing before the invention of the MICR line, and still serves as a backup in check processing should the MICR line become illegible or torn; it generally appears in the upper right part of a check near the date.

The MICR number is of the form

XXXXYYYYC
where XXXX is Federal Reserve Routing Symbol, YYYY is ABA Institution Identifier, and C is the Check Digit, while the fraction is of the form:

PP-YYYY/XXXX
where PP is a 1 or 2 digit Prefix, no longer used in processing, but still printed. Sometimes a branch number or the account number are printed below the fraction form; branch number is not used in processing, while the account number is listed in MICR form at the bottom. Further, the Federal Reserve Routing Symbol and ABA Institution Identifier may have fewer than 4 digits in the fraction form. The essential data, shared by both forms, is the Federal Reserve Routing Symbol (XXXX), and the ABA Institution Identifier (YYYY), and these are usually the same in both the fraction form and the MICR, with only the order and format switched (and left-padded with 0s to ensure that they are 4 digits long).

The prefix and the Federal Reserve Routing Symbol (XXXX) are determined by the bank's geographical location and treatment by the Federal Reserve type, while the remaining data (YYYY, and Branch number, if present) depends on the specific bank, and are unique within a Federal Reserve district.

In the check depicted above right, the fraction form is 11-3167/1210 (with 01 below it) and MICR form is 129131673 which are analyzed as follows:

the prefix 11 corresponds to San Francisco,
3167 (common to both) is the ABA Institution Identifier,
1210 and 1291 are the Federal Reserve Routing Symbols (generally equal, here different probably due to obfuscation, see image file history for more information), with the initial "12" corresponding to the Federal Reserve Bank of San Francisco, the third digits ("1" and "9") corresponding to check processing centers, and the fourth digits ("0" and "1") corresponding to where the bank is located – "0" indicates "in the Federal Reserve city of San Francisco", while "1" indicates "in the state of California".
the final "3" in the MICR is the check digit, and
the "01" below the fraction form is the branch number.
In the case of a MICR line that is illegible or torn, the check can be still be processed without the check digit. Typically, a repair strip or sleeve is attached to the check, then a new MICR line is imprinted. Either 021200025 or 0212-0002 (with a hyphen, but no check digit) may be printed, and both are 9 digits. The former (with check digit) is preferred to ensure better accuracy, but requires computing the check digit, while the latter is easily determined by inspection of the fraction, with minimal clerical handling.


MICR Routing number format

The MICR routing number consists of 9 digits:

XXXXYYYYC
where XXXX is Federal Reserve Routing Symbol, YYYY is ABA Institution Identifier, and C is the Check Digit.

Federal Reserve Routing SymbolThe Federal Reserve Routing Symbol were originally assigned in the systematic way outlined below, reflecting a bank's geographical location and internal handling by the Federal Reserve. However, the link is today tenuous – following banking consolidation, many banks use a routing number from a now-defunct bank, while the Federal Reserve no longer assigns specific numbers for thrifts, nor does the "check processing facility" have any current meaning, as check processing is now centralized within each Federal Reserve district.[

First two digitsThe first two digits of the nine digit ABA number must be in the ranges 00 through 12, 21 through 32, 61 through 72, or 80.

The digits are assigned as follows:

00 is used by the United States Government
01 through 12 are the "normal" routing numbers, and correspond to the 12 Federal Reserve Banks. For example, 0260-0959-3 is the routing number for Bank of America incoming wires in New York, with the initial "02" indicating the Federal Reserve Bank of New York.
21 through 32 were assigned only to thrift institutions (e.g. credit unions and savings banks) through 1985, but are no longer assigned (thrifts are assigned normal 01–12 numbers). Currently they are still used by the thrift institutions, or their successors, and correspond to the normal routing number, plus 20. (For example, 2260-7352-3 is the routing number for Grand Adirondack Federal Credit Union in New York, with the initial "22" corresponding to "02" (New York Fed) plus "20" (thrift).)
61 through 72 are special purpose routing numbers designated for use by non-bank payment processors and clearinghouses and are termed Electronic Transaction Identifiers (ETIs), and correspond to the normal routing number, plus 60.
80 is used for traveler's cheques

The first two digits correspond to the 12 Federal Reserve Banks as follows: