Although the number of digits in a checking account varies by bank, most use from nine to 12, according to U.S. Bank. Banks print the checking account number as the second set of digits in the series of magnetic numbers at the bottom left of the check.
The bank routing number and checking account direct the check through the banking clearinghouse that returns the instrument to the customer's bank. Citizen's Bank indicates financial institutions require these numbers when a checking account holder sets up direct deposits or electronic withdrawals using his checking account. The routing number is unique to the checking institution, while the account number is unique within the bank for the particular account.