What are the penalties if I file my US tax return too late or pay my US tax too late?

Penalties (US)

Late filing

If you do not file your return by the due date (including extensions), the penalty is usually 5% of the amount due for each month or part of a month your return is late, unless you have a reasonable explanation. If you do, include it with your return. The penalty can be as much as 25% of the tax due. The penalty is 15% per month, up to a maximum of 75%, if the failure to file is fraudulent. If your return is more than 60 days late, the minimum penalty will be $135 or the amount of any tax you owe, whichever is smaller.

Late payment of tax

If you pay your taxes late, the penalty is usually 1⁄2 of 1% of the unpaid amount for each month or part of a month the tax is not paid. The penalty can be as much as 25% of the unpaid amount. It applies to any unpaid tax on the return. This penalty is in addition to interest charges on late payments.

Frivolous return

In addition to any penalties, the law imposes a penalty of $5,000 for filing a frivolous return. A frivolous return is one that does not contain information needed to figure the correct tax or shows a substantially incorrect tax because you take a frivolous position or desire to delay or interfere with the tax laws. This includes altering or striking out the preprinted language above the space where you sign.

Other penalties can be imposed for negligence, substantial understatement of tax, reportable transaction understatements, filing an erroneous refund claim, and fraud. Criminal penalties may be imposed for willful failure to file, tax evasion, or making a false statement.