IRS Accuracy-Related Penalty for Substantial Understatement

The IRS accuracy-related penalty for a substantial understatement of income tax adds 20 percent to the portion of your underpayment caused by understating what you owed. Under Internal Revenue Code Section 6662, an understatement becomes “substantial” for an individual when it exceeds the greater of 10 percent of the correct tax or $5,000.¹ The penalty stacks on top of the tax you already owe, and interest runs on both from the original due date of the return.

When an Understatement Counts as Substantial

An understatement is the gap between the tax shown on your return and the tax you should have reported. For an individual, that gap crosses into “substantial” territory when it beats the greater of 10 percent of the correct tax or $5,000. Suppose the correct tax was $60,000 and you reported $48,000. The $12,000 shortfall clears the 10 percent threshold ($6,000) and triggers the penalty. Reverse the numbers slightly — correct tax of $30,000, understatement of $4,500 — and neither the percentage nor the $5,000 floor is met, so this trigger doesn’t apply.

Corporations other than S corporations and personal holding companies work off a different formula. A corporate understatement is substantial if it exceeds the lesser of 10 percent of the correct tax (or $10,000, whichever is larger) or $10 million.²

How the 20 Percent Is Calculated

The penalty is a flat 20 percent of the underpayment tied to the substantial understatement.¹ If an audit determines you underpaid by $15,000, the penalty is $3,000, bringing your additional liability to $18,000 before interest.

Interest runs alongside. The IRS charges interest on both the unpaid tax and the penalty from the return’s original due date, and it accrues daily until the balance is paid.³ An extension to file does not extend the time to pay, so the interest clock does not pause while you sort out the return. On a dispute that drags through audit and appeals, the interest itself can become a meaningful part of the bill.

Shrinking the Understatement Before the Penalty Applies

Not every dollar of understatement counts toward the penalty. The law lets you reduce it by any portion attributable to a tax position backed by substantial authority, or by any item you adequately disclosed on your return that also has a reasonable basis.¹ Take enough off the top and the remaining understatement may drop below the “substantial” threshold entirely.

Substantial Authority

Substantial authority is an objective test. You weigh the legal sources supporting your position against those opposing it. If the weight of authority meaningfully favors your position, the standard is met. It sits above “reasonable basis” but below “more likely than not” (which requires better than a 50 percent chance of winning). The advantage is that substantial authority protects you without any special disclosure on the return.

Disclosure With a Reasonable Basis

If your position doesn’t rise to substantial authority, disclosure is the fallback. Disclosure requires at least a reasonable basis, a standard the IRS describes as significantly higher than “not frivolous.” You disclose on Form 8275, attached to your original return; an informal letter or memo won’t count.⁴ If your position directly contradicts a Treasury regulation, use Form 8275-R, which requires a detailed explanation of why you believe the regulation is invalid.⁵

Disclosure has boundaries. It won’t shield you from penalties for negligence, valuation misstatements, transactions lacking economic substance, or any understatement tied to a tax shelter. It also won’t help if you failed to keep adequate books and records to support the position.⁴

Fixing the Error With a Qualified Amended Return

One of the cleanest ways to eliminate the penalty is to correct the return before the IRS finds the problem. A “qualified amended return” increases the tax shown on the original return, which directly reduces the underpayment the penalty attaches to. Correct the full understatement and there is nothing left for the penalty to apply against.

Timing is everything. The amended return must be filed before the earliest of several triggering events, the most common being the date the IRS first contacts you about examining that return. Once you get an audit letter, the window is closed. Other triggers include the IRS contacting a pass-through entity connected to the return, service of certain summonses related to your tax liability, or the Commissioner announcing a settlement initiative for a listed transaction you took part in.⁶ This protection is not available for fraudulent positions on the original return. You will still owe the additional tax and interest, but avoiding the 20 percent penalty is often significant.

The Reasonable Cause and Good Faith Defense

Once the penalty has been proposed or assessed, the main statutory defense is reasonable cause and good faith. Section 6664 states that no penalty applies to any portion of an underpayment where you can show both.⁷ The IRS evaluates the defense by asking whether you exercised the care a reasonably prudent person would exercise and still got it wrong.⁸

Examiners want to know what happened, why it prevented compliance, how it affected the rest of your affairs, and what you did to fix the problem once the obstacle passed.⁸ Circumstances that commonly support reasonable cause include:

Death, serious illness, or unavoidable absence affecting you, an immediate family member, or (for a business) the person with sole authority over the return.
Fire, natural disaster, or casualty that destroyed records or made compliance impossible despite your efforts.
Inability to obtain records for reasons outside your control.
Reliance on the advice of a qualified tax professional who had complete and accurate facts.

Reliance on a professional is scrutinized closely. The IRS will look at whether the advisor was actually qualified, whether you supplied full and accurate information, and whether the advice was reasonable on its face. Dropping off a bag of receipts and hoping for the best does not qualify.

Tax Shelter and Reportable Transaction Limits

If the understatement involves a tax shelter, the standard reductions for substantial authority and adequate disclosure don’t apply. A tax shelter for this purpose is any partnership, entity, investment plan, or arrangement where a significant purpose is avoiding federal income tax.¹ For reportable transactions, reasonable cause survives only if you adequately disclosed the transaction, had substantial authority for the treatment, and reasonably believed your position was more likely than not correct. Opinions from disqualified advisors — promoters, contingent-fee advisors, or those with a financial stake in the outcome — don’t count.⁹

Responding to a Penalty Notice

The IRS typically proposes the accuracy-related penalty during an audit or through a notice such as the CP2000, which flags mismatches between your return and information the IRS received from employers, banks, and brokers.¹⁰ Your reply deadline is printed on the notice. Respond by that date, state clearly whether you agree or disagree, and attach any documentation supporting your position.

To request relief after the penalty has been assessed, you can call the IRS for straightforward situations or submit a written request on Form 843.¹¹ Identify the penalty, the tax period, and your specific grounds. Attach anything that supports reasonable cause: professional advice you relied on, medical records, disaster documentation, bank records contradicting the IRS’s findings. Keep copies of everything you send and record the mailing date.

One misconception worth clearing up: the IRS First Time Abate program, which waives penalties for taxpayers with a clean compliance history, generally covers failure-to-file and failure-to-pay penalties. It does not cover accuracy-related penalties. Reasonable cause is the primary path to relief from a Section 6662 penalty.

Appealing a Denial

If the IRS rejects your abatement request, you generally have 30 days from the date of the rejection letter to request a conference with the IRS Independent Office of Appeals.¹² The rejection letter states the exact deadline. Appeals officers are independent from the examination function and take a fresh look at the facts, so outcomes there can differ from the initial decision.

If the dispute reaches a formal notice of deficiency (sometimes called a 90-day letter), you have 90 days from the date on the notice to petition the U.S. Tax Court, or 150 days if you are outside the country. The Tax Court offers simplified procedures for disputes of $50,000 or less per tax year, which covers many individual penalty cases.¹³ Filing in Tax Court lets you challenge the penalty without paying first. Miss the 90 days and your remaining option is to pay, file a refund claim, and sue in federal district court or the Court of Federal Claims.

How Long the IRS Has to Assess

The accuracy-related penalty follows the same clock as the underlying tax. Ordinarily the IRS has three years from the date you filed the return (or the due date, whichever is later) to assess additional tax and penalties.¹⁴ That window stretches to six years when you omitted more than 25 percent of your gross income.¹⁵ There is no time limit for fraudulent returns or years when no return was filed.

During an audit, the IRS may ask you to sign Form 872 or a similar agreement extending the assessment period. You can negotiate the length or refuse to sign, though refusing may prompt the IRS to issue a notice of deficiency to protect its deadline rather than continue working with you cooperatively.¹⁴

1
Office of the Law Revision Counsel. 26 USC 6662 – Imposition of Accuracy-Related Penalty on Underpayments
2
GovInfo. 26 USC 6662 – Substantial Understatement of Income Tax
3
Internal Revenue Service. Interest
4
Internal Revenue Service. Instructions for Form 8275
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Internal Revenue Service. Instructions for Form 8275-R
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eCFR. 26 CFR 1.6664-2 – Underpayment
7
Office of the Law Revision Counsel. 26 USC 6664 – Definitions and Special Rules
8
Internal Revenue Service. IRM 20.1.1 Introduction and Penalty Relief
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Office of the Law Revision Counsel. 26 USC 6664 – Definitions and Special Rules
10
Internal Revenue Service. Understanding Your CP2000 Series Notice
11
Internal Revenue Service. About Form 843, Claim for Refund and Request for Abatement
12
Internal Revenue Service. Penalty Appeal
13
Internal Revenue Service. Understanding Your CP3219N Notice
14
Internal Revenue Service. Time IRS Can Assess Tax
15
Office of the Law Revision Counsel. 26 USC 6501 – Limitations on Assessment and Collection