IRC 6651(a)(3): The Failure-to-Pay Penalty on Assessed Deficiencies

When an audit, an underreporter notice, or a math error adds tax to your return, the bill comes with its own failure-to-pay penalty rules. They are kinder than the rules for tax you showed on the return, at least at first. Interest is not kind at all. It reaches back to the original due date.

Two different failure-to-pay penalties

IRC 6651 has two failure-to-pay provisions, and they cover different dollars:

The mechanics of the first column are in the failure-to-pay guide. This guide is about the second.

The statute

IRC 6651(a)(3) imposes the penalty when any amount of tax required to be shown on a return, but not shown (including a math error assessment under IRC 6213(b)), is not paid within 21 calendar days from the date of notice and demand, or 10 business days if the amount is $100,000 or more. The rate is 0.5 percent of the tax stated in the notice for the first month, plus 0.5 percent for each additional month or fraction, capped at 25 percent.

IRC 6651(b)(3) reduces the base for each month by any part of the tax paid before the beginning of that month. Same monthly recomputation as the regular failure-to-pay penalty.

Counting the grace period and the months

IRM 20.1.2.3.8.5.2 says the notice states a payment date. For notices under $100,000, that date is 21 calendar days after the notice date. For $100,000 or more, it is 10 business days after. The first penalty month begins the day after that payment date. Each later month ends on the same day of the month.

Worked example: CP2000 assessment

You filed your 2024 return on time and paid what it showed. In 2026, an underreporter case adds $8,000 of tax. The assessment notice and demand is dated March 2, 2026, so the payment date is March 23. You pay the $8,000 in full on September 30, 2026.

Penalty months begin March 24, April 24, May 24, June 24, July 24, August 24, and September 24. September 30 falls in the seventh month.

Now the interest

Here is where the timing difference bites. Under IRC 6601(a), interest runs from the last date prescribed for payment, which for the 2024 tax is April 15, 2025. Not from the date the IRS found the problem. Not from the notice. Compounded daily under IRC 6622 at the quarterly rates in Rev. Rul. 2026-15: 7 percent from April 2025 through March 2026, 6 percent for April through June 2026, and 7 percent for July through September 2026.

The penalty was $280. The interest was $838.86, and most of it accrued before the IRS ever sent the bill. The grace period protects you from the penalty. It does nothing about interest that already happened.

Paying within the grace period

If you pay the full amount within 21 calendar days (or 10 business days for $100,000 or more), there is no IRC 6651(a)(3) penalty at all. IRC 6601(e)(3) adds a second benefit: interest on the amount paid stops accruing after the date of the notice and demand. So paying on day 20 costs the same interest as paying on the notice date.

Large notices: the 10-business-day trap

A $150,000 assessment dated March 2, 2026 has a payment date of March 16, 2026. That is two weeks, not three. Businesses and high-income taxpayers get the shorter window precisely when the amounts are biggest. Calendar it the day the notice arrives.

Separate caps for separate notices

IRM 20.1.2.3.8.1 says each notice reflecting additional tax gets its own penalty computation and its own 25 percent cap. The original balance from the return can be capped out while the penalty on a later audit assessment is still in month three. When you check a transcript, compute each assessment separately.

Late returns: failure to file applies to the deficiency too

If the original return was late, the failure-to-file penalty is based on the tax required to be shown, not just what you reported. When an audit increases the tax, the IRS recomputes the failure-to-file penalty on the higher number. IRM 20.1.2.3.8.7.3 shows exactly that: a deficiency notice that included both the extra tax and an additional failure-to-file amount. And under IRC 6601(e)(2)(B), interest on that failure-to-file addition runs from the return due date. See interest on penalties.

What about the accuracy-related penalty?

Audit assessments often carry a 20 percent accuracy-related penalty under IRC 6662. That is a separate penalty, computed on the underpayment, with its own interest start date. The math is in the accuracy-related penalty guide. If an audit result looks wrong, the time to challenge it is before assessment; see IRS Audit: What to Expect.

Checking an audit bill

1. Find the assessment of additional tax on the transcript (often TC 290 or TC 300) and the date of the notice and demand.
2. Add 21 calendar days, or 10 business days if the notice amount is $100,000 or more.
3. Count penalty months from the next day. Charge 0.5% on the unpaid additional tax at the start of each month, or 1% for months beginning more than 10 days after a levy notice.
4. Separately, compute interest on the additional tax from the original return due date to the payment date.

Worked example: a levy notice on top of an audit bill

Return to the $8,000 assessment with notice and demand dated March 2, 2026, and penalty months that begin on the 24th. This time the bill is not paid, and a notice of intent to levy is dated June 10, 2026. Under IRC 6651(d), the trigger day is 10 days later, June 20. The penalty month that began May 24 started before the trigger, so it stays at 0.5 percent. The month beginning June 24 is the first at 1 percent.

If the $8,000 is paid on September 30, 2026, the months beginning March 24, April 24, and May 24 cost $40 each, or $120. The months beginning June 24, July 24, August 24, and September 24 cost $80 each, or $320. The penalty is $440 instead of the $280 it would have been without the levy notice. That is the same method the IRS uses in its own deficiency example at IRM 20.1.2.3.8.7.3, where the rate increases for penalty months beginning on or after the 11th day after the notice.

Interest is unaffected by the levy notice. It still runs from April 15, 2025 on the 2024 deficiency, and it was $838.86 through September 30, 2026 in the earlier computation.

Partial payments inside the grace period

Paying part of the bill within the 21 days still helps. Under IRC 6651(b)(3), the base for each penalty month is the tax stated in the notice reduced by tax paid before the beginning of that month. Say you pay $5,000 of the $8,000 by March 20, 2026, inside the window, and the remaining $3,000 on September 30. The penalty runs on $3,000 for seven months at 0.5 percent, or $105, instead of $280. And under IRC 6601(e)(3), no interest is charged after the notice date on the $5,000 you paid within the window.

The same logic applies to math error assessments under IRC 6213(b). The additional tax from a math error is billed on a notice and demand, and the 21-day and 10-business-day windows apply in the same way.

The bottom line

On an audit or CP2000 bill, the failure-to-pay penalty waits 21 days. Interest never waited; it has been running since the original due date. Pay inside the grace period if you can. If you cannot, know which clock is which before you plan around it.