What is the penalty for filing taxes late?

If you owe money, the failure-to-file penalty is 5% of unpaid taxes per month, up to 25%. If you're getting a refund, there's no penalty for filing late — but you have 3 years to claim it before the IRS keeps your refund. Always file an extension (Form 4868) if you can't make the April deadline; it gives you until October 15.