If you owe tax, how does the IRS failure-to-file penalty work?
Here are some facts about that from IRS.gov.
1 If you do not file by the deadline, you might face a failure-to-file penalty.
2 If you do not pay by the due date, you could face a failure-to-pay penalty.
3 The failure-to-file penalty is generally more than the failure-to-pay penalty. So if you cannot pay all the taxes you owe, you should still file your tax return and explore other payment options in the meantime.
4 The penalty for filing late is usually 5 percent of the unpaid taxes for each month or part of a month that a return is late. This penalty will not exceed 25 percent of your unpaid taxes.
5 If you file your return more than 60 days after the due date or extended due date, the minimum penalty is the smaller of $135 or 100 percent of the unpaid tax.
6 You will have to pay a failure-to-pay penalty of ½ of 1 percent of your unpaid taxes for each month or part of a month after the due date that the taxes are not paid. This penalty can be as much as 25 percent of your unpaid taxes.
7 If you filed an extension and you paid at least 90 percent of your actual tax liability by the due date, you will not be faced with a failure-to-pay penalty if you file by the extended due date and pay the remaining balance with your return.
8 If both the failure-to-file penalty and the failure-to-pay penalty apply in any month, the 5 percent failure-to-file penalty is reduced by the failure-to-pay penalty. However, if you file your return more than 60 days after the due date or extended due date, the minimum penalty is the smaller of $135 or 100% of the unpaid tax.
9 You will not have to pay a failure-to-file or failure-to-pay penalty if you can show that you failed to file or pay on time because of reasonable cause and not because of willful neglect.
There is a lot more info about this on their site.