ABA Routing Number Validator

In the United States a nine-digit number identifies the bank that holds an account, and it appears at the bottom left of every cheque, in direct-deposit forms and in payment files. It is called the ABA routing transit number after the American Bankers Association, which created the scheme in 1910. Typing one wrongly can send a payment to the wrong bank, so the number includes a checksum. This validator evaluates it and reads the structure.

The checksum

Number the digits d1 to d9. Multiply d1, d4 and d7 by 3, d2, d5 and d8 by 7, and d3, d6 and d9 by 1, and add everything. If the total is a multiple of 10, the number is well formed. The ninth digit is the check digit: it is chosen so that the total comes out right. That is a “3-7-1” weighting. It catches any single wrong digit and many transpositions, although a swap of two neighbouring digits that differ by exactly 5 goes unnoticed.

A worked example

Press Sample to check six numbers. 011000015 is valid: weighted sum 20, Federal Reserve routing symbol 0110, which is the Federal Reserve Bank of Boston (district 1), commercial bank, ABA institution identifier 0001, check digit 5. 021000021 is valid with weighted sum 30 in the New York district (2). 322271627 has a weighted sum of 120, and the symbol 3222 marks a thrift institution in district 12 (San Francisco). 121000248 has a sum of 60 and is a commercial bank in district 12. 011000016 changes only the last digit: the weighted sum is 21, so it fails, and the page shows that the correct check digit is 5. 12345 has five digits and is rejected on length.

Reading the digits

The first digit group works like a map. The first two digits show the Federal Reserve district when they are 01 to 12, a thrift when they run 21 to 32, and electronic payments such as ACH when they fall in 61 to 72. Digits three and four identify the Federal Reserve office that handles the bank’s cheques. The next four digits are the ABA institution identifier, assigned to the bank, and the last is the check digit. The page spells out the district name and institution type, so a number that claims to be a commercial bank in New York but decodes to a thrift in San Francisco stands out.

Using the result

Pass the numbers in a direct-deposit file through the validator before upload. If a number fails, correct it with the digit the page suggests only after you confirm it with the bank, since the suggested digit proves only the arithmetic. If it passes, confirm the bank and the transfer type with the Federal Reserve’s E-Payments Routing Directory or by phone.

Limits

A correct checksum does not prove the number belongs to an operating bank, nor which payment types it handles. About one in ten random nine-digit numbers passes the arithmetic. The page has no bank directory and sends nothing.

Related tools

• IBAN Validator is the international equivalent for account numbers.
• SWIFT/BIC and IFSC Decoder validates bank codes for international transfers.
• Luhn Validator checks card numbers with another formula.
• GTIN, EAN and UPC Check Digit Calculator shows a similar weighted-sum check for barcodes.