If you do not file your federal income tax on time and are due a refund, there is no penalty assessed by the IRS. However, if you do owe taxes and file late, it is to your advantage to file as promptly as possible. The late filing penalty is 5% of the additional taxes owed amount for every month (or fraction thereof) your return is late, up to a maximum of 25%. If you file more than 60 days after the due date, the minimum penalty is $135 or 100% of your unpaid tax, whichever is smaller.