You have 3 years to file a return if you are expecting a refund. If you don't file by then, the U.S. Treasury simply keeps your "donation."
If you owe taxes, it's recommended to file as soon as possible.
The IRS applies late penalties and interest on a case-by-case basis and will send a separate bill if penalties apply.
Because the IRS has the last word on penalties, we can't calculate the exact amount if your return is late. But the info below will give you an idea of what to expect in a worst-case scenario (courtesy of IRS Tax Topic 653).
No penalty if you're getting a tax refund.
However, you must file your 2017 taxes by April 18, 2021 (or October 16 of 2021, if you filed an extension). After that, any unclaimed tax refunds get turned over to the U.S. Treasury.
No penalty if you file by October 16, 2018, provided you filed an extension and paid your tax bill by April 17, 2018.
Late filing penalties apply if you owe taxes and didn't file your return or extension by April 17, 2018, or if you filed an extension but failed to file your return by October 16, 2018.
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 $205 or 100% of your unpaid tax, whichever is less.
Late payment penalties apply if you didn't pay taxes owed by April 17, 2018, regardless of whether you filed an extension or not.
The late payment penalty is 0.5% (1/2 of 1 percent) of the additional tax owed amount for every month (or fraction thereof) the owed tax remains unpaid, up to a maximum of 25%.
For any month(s) in which both the late-payment and late-filing penalties apply, the 0.5% late-payment penalty is waived.
Interest (compounded daily) starts accumulating on unpaid taxes one day after the due date of the return, until the bill is fully paid off. The current interest rate is 4.18% (3% on top of the federal short-term rate of 1.18%) and is subject to change.