How is the IRS late filing penalty interest calculated ?

Interest is charged on any unpaid tax due from the due date of your tax return (usually April 15) until the date of payment. The interest rate is determined every 3 months. There are separate late filing and late paying penalties. If you file on time but do not pay on time, you will generally have to pay a penalty of one half of 1% of the unpaid tax due for each month or part of a month the tax remains unpaid. If you owe tax and don't file on time, the penalty for not filing on time will be separately calculated.