Routing numbers are 9-digit codes that identify US financial institutions for electronic transactions. Understanding their structure helps validate them correctly.
Routing Number Structure
The 9 digits are structured as follows: digits 1-4 are the Federal Reserve routing symbol, digits 5-8 are the ABA institution identifier, and digit 9 is the check digit used for validation.
Check Digit Validation
The 9th digit is calculated using a specific algorithm. Multiply digits by weights (3,7,1,3,7,1,3,7,1), sum the results, and the total should be divisible by 10. This catches most transcription errors.
Types of Routing Numbers
Banks may have different routing numbers for paper (checks) vs. electronic (ACH/wire) transactions. Always verify you are using the correct type for your transaction.