IRS penalties vary depending on your circumstances, so it’s important not to assume that the IRS late penalties will be the only penalties you pay when you file late.
That being said, the IRS penalty for filing late is an additional fee in the amount of 5% of tax due for every month that you don’t file your income tax return, up to 25%. The clock on the penalty starts ticking the day that your tax return is due (either May 17th, or October 15th if you filed an extension). However, if your return is more than 60 days late, the minimum penalty is either $100 or 100% of the amount of tax due, whichever is smaller. You may see a similar penalty for your state return, depending on your state.
The penalty for paying late is .5% of tax due for every month that you don’t file, up to 25%. Both the late filing and late payment penalties can be applied at the same time.
On top of these penalties, the IRS will also charge interest on the tax that you owe. Interest starts accruing on the due date of your return, and is generally the same as the federal short-term interest rate plus 3%.