When Interest Starts on IRS Penalties: The IRC 6601(e)(2) Rules

Interest on tax is easy: it starts on the original payment due date. Interest on penalties is not one rule. It is two, and which one applies depends on which penalty you are looking at. Getting this wrong is the most common reason a hand calculation of an IRS balance misses.

The general rule: after notice and demand

IRC 6601(e)(2)(A) says interest is imposed on an assessable penalty, additional amount, or addition to tax only if it is not paid within 21 calendar days from the date of notice and demand (10 business days if the amount is $100,000 or more). If it is not paid in that window, interest runs from the date of the notice and demand to the date of payment.

Two details matter. First, the grace period is a condition, not a delay: miss it and interest is charged from the notice date, not from day 22. Second, until the IRS actually assesses and bills a penalty, there is no notice and demand, so these penalties draw no interest while they are still accruing.

The exception: from the return due date

IRC 6601(e)(2)(B) carves out a group of additions that draw interest from the date the return is required to be filed, including extensions, until paid. The group is:

• the failure-to-file addition under IRC 6651(a)(1),
• additions under IRC 6653, and
• penalties under part II of subchapter A of chapter 68, which includes the accuracy-related penalty under IRC 6662 and the civil fraud penalty under IRC 6663.

Worked example: one late return, two penalty clocks

A 2025 Form 1040 was due April 15, 2026, with $6,000 owed. No extension. The return arrives August 3, 2026, without payment. The IRS sends notice and demand dated September 14, 2026, billing a failure-to-file penalty of $1,080 and $150 of failure-to-pay penalty accrued so far. Everything is paid December 31, 2026.

The 21-day window from September 14 ends October 5, 2026. Nothing was paid by then, so the failure-to-pay penalty draws interest from September 14.

The small numbers make the point cleanly. The failure-to-file penalty drew interest for 260 days. The failure-to-pay penalty drew interest for 108. Same return, same payment date.

Worked example: an accuracy penalty years later

Where the due-date rule really bites is the accuracy-related penalty, because audits come years after the return. Say the IRS audits a 2023 return (due April 15, 2024) and in 2026 assesses a $10,000 accuracy-related penalty. The taxpayer pays on September 30, 2026.

About two and a half years of interest on a penalty that did not exist until the audit ended. The rule is deliberate. Congress decided penalties tied to the return itself should carry interest from when the return was due. The math behind the penalty is in the accuracy-related penalty guide.

Paying inside the grace period

IRC 6601(e)(3) adds a related rule for any amount billed in a notice and demand: if it is paid within 21 calendar days (10 business days for $100,000 or more), no interest is imposed on the amount paid for the period after the notice date. For a failure-to-pay penalty, that means paying inside the window produces zero interest on the penalty. For a failure-to-file penalty, it freezes interest at the notice date.

Absent a designation, voluntary payments are generally applied to tax first, then penalties, then interest. The rules and the strategy are in how the IRS applies payments.

Abatement removes the penalty's interest too

If a penalty is abated, the interest that accrued on it goes with it. For a failure-to-file or accuracy-related penalty, that can be years of interest. See recomputing the balance after abatement.

Reading this on a transcript

• The failure-to-file penalty usually appears with the return assessment. Its interest is folded into the interest the IRS computes for the module.
• Failure-to-pay penalty is assessed in pieces over time as notices go out. Each piece starts its own interest clock only from its own notice date.
• IRS interest figures on notices are computed to a specific date. Recompute to the same date before comparing.

For the compounding method itself, see how IRS interest compounds daily.

Worked example: an extension moves the start date

For the failure-to-file and accuracy-related penalties, IRC 6601(e)(2)(B) starts interest on the date the return is required to be filed, including extensions. Interest on the tax itself starts on the original payment date, extensions or not. On an extended return, that creates two different start dates on the same account.

Say a 2024 return was on a valid extension to October 15, 2025. An audit later assesses a $10,000 accuracy-related penalty, paid September 30, 2026. Interest on that penalty runs from October 15, 2025, and at the quarterly rates in Rev. Rul. 2026-15 it comes to $667.58. If the same penalty had been on a return with no extension, interest would have run from April 15, 2025, and come to $1,048.58. The extension did not change the penalty, but it moved the interest start date six months later and saved $381.

Worked example: paying the deposit penalty inside 21 days

The failure-to-deposit penalty under IRC 6656 follows the general rule in 6601(e)(2)(A). Say a business receives a notice dated September 8, 2026 billing a $2,040 deposit penalty. The 21-day window closes September 29. Paid on September 25, there is no interest on the penalty at all. Paid on October 20, interest runs from September 8, the notice date, not from day 22. At 7 percent compounded daily, that is $16.50.

The number is small, but the rule is the point. Missing the window by one day does not cost one day of interest; it costs interest back to the notice date.

Which penalty to pay first

If you can pay only part of what you owe, these start dates suggest an order. The failure-to-file and accuracy-related penalties have usually been accruing interest the longest, often from the return due date. A failure-to-pay penalty billed last month has barely started. Designating a voluntary payment to the older penalty stops more interest per dollar. Without a designation, Rev. Proc. 2002-26 applies a partial payment within a period to tax, then penalty, then interest, so you need the written designation to pick among penalties.

• Tax itself always draws interest from the original payment date under IRC 6601(a).
• Failure-to-file, accuracy-related, and civil fraud penalties draw interest from the return due date including extensions.
• Most other penalties draw interest only after a notice and demand goes unpaid for 21 days, or 10 business days for $100,000 or more.
• Estimated tax penalties draw no interest under IRC 6601(h).

When you reconstruct a balance, keep a separate interest line for each penalty assessment, with its own start date, rather than one combined penalty figure. That is the only way to match the IRS's totals and the only way to know what a partial payment will actually stop.

The bottom line

Failure-to-file, accuracy-related, and fraud penalties draw interest from the return due date. Most other penalties draw interest only after they are billed and left unpaid for 21 days. When you rebuild an IRS balance, give each penalty its own start date. When you decide what to pay first, that same rule should drive the order, and it is worth a professional conversation if the dollars are real.