ABA Routing Number Validator

How it is calculated

Type a routing number; every character that is not a digit (spaces, hyphens) is removed first, so 0210-00021 and 021000021 are treated the same. If exactly nine digits remain, each digit is multiplied by a repeating weight pattern 3, 7, 1, 3, 7, 1, 3, 7, 1 and the products are added: sum = 3(d1+d4+d7) + 7(d2+d5+d8) + 1(d3+d6+d9). The number passes when sum mod 10 = 0. The tool shows the digit count, the sum and a valid flag, where 1 means the checksum passes and 0 means it fails. With any other number of digits, the sum is shown as a dash and the flag is 0. A text with no digits at all gives no result. This tests the check digit only: it does not look the number up, so a passing number may still not belong to any bank, and even a string of nine zeros passes.

Common mistakes

• Reading a valid flag of 1 as confirmation that the bank exists. The check only catches typing errors; the number must still be looked up in the official register of routing numbers.
• Entering an account number, a SWIFT/BIC code or an IBAN. Only the 9-digit ABA routing number is meant, and a longer string just reports the wrong length.
• Dropping the leading zero. Routing numbers often start with 0, and an 8-digit entry fails on length before any checksum is calculated.
• Expecting a character-by-character error report. When the sum is not a multiple of 10 the tool says only that it fails, not which digit is wrong.
• Using it for wire details outside the U.S. Routing numbers of this form belong to the U.S. system; other countries use different formats and check rules.