There are basically three penalty-like charges when you don’t file or pay on time:
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Failure to file (FTF) penalty. This is 5% of the the unpaid tax shown on your return per month or fraction of a month until the return is filed. Maximum 25%. Note that this penalty does not stop accruing until you file your return. Making a late payment of the amount due does not reduce the penalty.
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Failure to Pay (FTP) penalty. This is 0.5% of the tax due per month or fraction of a month until the tax is paid.
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Interest. Interest accrues on a daily basis on the unpaid balance, including the unpaid penalties. The current annualized rate is 4%, but this can change quarterly.
If you have an extension, the FTF penalty doesn’t start until the extended due date.
If you have an extension, the FTP penalty does not start until the extended due date as long as you have paid at least 90% of your tax due by the regular due date. Otherwise, it starts on the regular due date.
Interest always accrues on payments not made by the regular due date until the balance is paid.
In any month where both the FTF and the FTP penalty apply, the FTF penalty is reduced so that the total is not more than 5%.
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But wait, there’s more than just penalties and interest. There are certain elections that can only be made on a timely filed return. If you miss the due date and don’t have an extension, you are simply out of luck.
A common one is the right to recharacterize an IRA contribution or to receive a return of an excess contribution without penalty. The deadline is the tax return due date including extensions. For this purpose you are considered to have a 6-month extension if you file on time.
You’d be surprised how many people screw up their IRA contributions and need to fix them. Many of them don’t figure out what they did wrong until after April 15th. But because somebody on the internet told them “You don’t have to file on time if you have a refund coming,” they lose their opportunity to fix it.
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And one more thing that all of the tax brains on the internet don’t tell you: Willful Failure to File a return when due is a criminal matter. 26 USC 7203.
You could be fined and/or sent to jail for willfully not filing your return on the due date. Now, the IRS does not routinely refer these cases for prosecution. In fact, they rarely do. Only when they need to make an example of you or when there are extreme circumstances do they do this. For example, if you charge people to teach them that there is no law making you pay income taxes or you are a drug king pin or something like that.
But surely that does not apply if you have a refund coming or don’t owe any tax, right? Wrong. U.S. v. HAIRSTON. Richard P Hairston failed to file his tax returns. He was criminally charged. The judge refused to permit him to present evidence that he would have received a refund had he filed. He was convicted. He appealed. The appeals court upheld the judge’s ruling.
So you see, there are plenty of reasons to get an extension if you need one.
Ignore the people on the internet who tell you there is no problem if you file late or don’t file at all.