• The interest rate is the federal short-term rate plus 3 percent. That rate is determined every three months.
• For current interest rates, go to News Releases and Fact Sheets and find the most recent Internal Revenue release entitled Quarterly Interest Rates.

In addition, if you filed on time but didn’t pay on time, you’ll generally have to pay a late payment penalty.

• The late payment penalty is one-half of one percent of the tax (0.5%) owed for each month, or part of a month, that the tax remains unpaid after the due date, not exceeding 25 percent.
• You will not have to pay the penalty if you can show reasonable cause for the failure.
• The one-half of one percent rate increases to one percent if the tax remains unpaid after several bills have been sent to you and the IRS issues a notice of intent to levy.
• Currently, if you filed a timely return and are paying your tax via an installment agreement, the penalty is one-quarter of one percent for each month, or part of a month, that the installment agreement is in effect.

If you did not file on time and owe tax, you may owe an additional penalty for failure to file unless you can show reasonable cause.

• The combined penalty is 5 percent (4.5% late filing, 0.5% late payment) for each month, or part of a month, that your return was late, up to 25%.
• The late filing penalty applies to the net amount due, which is the tax shown on your return and any additional tax found to be due, as reduced by any credits for withholding and estimated tax and any timely payments made with the return.
• After five months, if you still have not paid, the 0.5% failure-to-pay penalty continues to run, up to 25%, until the tax is paid.
• The total penalty for failure to file and pay can be 47.5% (22.5% late filing, 25% late payment) of the tax owed.
• If your return was over 60 days late, the minimum failure-to-file penalty is the smaller of $100 ($135 for returns required to be filled after December 31, 2008) or 100% of the tax required to be shown on the return.