If you owe taxes and didn’t file your tax return or request an extension by the April 18 deadline, you may face interest on any unpaid federal taxes you owe and a failure-to-file penalty. The IRS will deny a request for an extension that is filed after 
midnight on April 18. However, you should still file your tax return, even if it’s late.

The failure-to-file penalty is 5 percent per month, or part of a month, of the balance due, up to a maximum of 25 percent. If the tax return is more than 60 days late, the minimum penalty is $135 or the balance due, whichever is less.

Interest and penalties add to the total amount you owe. The sooner you file, even if you can’t pay all or some of the taxes due, the less you will owe.

More information about penalty and interest charges is contained in Chapter 1, Filing Information, of IRS Publication 17.