What is the penalty for NOT filing your tax return by the deadline if you owe taxes?

The failure-to-file penalty is generally 5% of your unpaid tax for each month (or partial month) you're late, up to a maximum of 25%. This is separate from the failure-to-pay penalty (0.5% per month). If you can't pay what you owe, filing on time and arranging a payment plan dramatically reduces total penalties.