An ABA routing transit number (ABA RTN) used in the United States is a nine digit code, The ABA RTN appears in the fraction form and the MICR (magnetic ink character recognition) form. The two form give essentially the same information.
The MICR form like this: XXXXYYYYC, It must pass the Luhn Algorithm. Where XXXX Federal Reserve Routing Symbol, YYYY is ABA Institution Identifier, and C is the Check Digit.
Example number: 021302567 053902197 036001808 011600033