Without an active extension in place, the last day to file your tax return is the original due date for the return (refer to When are taxes due? ). If you are not able to complete your tax return by that date, be sure to file an extension before the original due date to add an additional six months to the deadline. It’s important to note, however, that an extension of time to file is not an extension of time to pay. In order to avoid an underpayment penalty, the government expects to have received the lesser of 90% of the taxes you owe by the end of that year or an amount equal to 100% of your tax liability for the previous year.
If for any reason you miss the original or extended deadline, it's always best to file as soon as possible in order to avoid penalties from accruing further. If you file your return more than 60 days after the due date or extended due date, the minimum penalty for late filing is the smaller of $135 or 100 percent of the unpaid tax. Penalty for late payment. The failure-to-pay penalty is generally 0.5 percent per month of your unpaid taxes.