Opening your mailbox and seeing “Internal Revenue Service” on the envelope is an instant stress spike. Heart races, stomach drops, brain goes straight to one word: audit.
But for the 2026 tax season, if you get a notice, odds are you are not being audited. In many cases, you are looking at a Math Error Notice, one of millions of automated letters the IRS sends every year when its computer system spots a discrepancy on your return.
These notices are usually less serious than an audit, but they are not harmless. They are what the IRS calls a summary assessment, which means your bill can change fast, and your rights to fight that change expire on a strict timeline.
This guide walks you through what those notices mean, how to read them, and what to do next.
What the IRS Really Means by a “Math Error”
When most of us hear “math error,” we think of a simple mistake. You added $200 and $300 and somehow got $600. Those mistakes count, but the IRS definition is much broader.
Under what is known as Math Error Authority (MEA), Congress lets the IRS correct certain “clerical” or obvious mistakes without going through a full audit. Common triggers include:
- Arithmetic mistakes: Basic addition, subtraction, or multiplication errors on the return.
- Data mismatches: A Social Security number or name for you or a dependent does not match what the Social Security Administration has on file.
- Missing information: You claimed a credit, such as the Earned Income Tax Credit (EITC), but did not include a required form or schedule.
- Exceeding legal limits: You claimed more of a credit or deduction than the law allows for your income or filing situation.
In short, if the IRS computer can clearly tell that an entry is wrong or incomplete based on what it already knows, it may “fix” your return on its own and then send you a notice showing the change. Sometimes that means a smaller refund, sometimes a bigger one, and sometimes a new balance due.
CP11, CP12, CP13: How To Decode Your Notice
To figure out exactly what changed, look in the top right corner of your letter. You will usually see one of these codes:
- CP11: The IRS found an error, corrected it, and now you owe money. Sometimes this means your refund was reduced so much that you now have a balance due.
- CP12: The IRS found an error, corrected it, and your refund amount changed. You may still be getting money back, just a different amount than you expected.
- CP13: The IRS found an error, corrected it, and the result is a zero balance. No money owed, no refund coming.
Inside the notice, you should see a side-by-side breakdown, usually labeled something like:
- “Figures from your return”
- “Figures we used”
That table is your roadmap. It shows exactly where the IRS changed your numbers.
New for 2026: The “IRS MATH Act”
If you have ever stared at a CP11 notice trying to decipher exactly what “calculation error” you made, you know the frustration. Historically, these notices were notoriously vague.
That is changing. In late 2025, the Internal Revenue Service Math and Taxpayer Help Act (H.R. 998) was signed into law. This legislation specifically targets the confusion around math error notices.
While full implementation is rolling out through late 2026, the IRS is moving toward these new Congressional standards. Under the new law, the IRS is required to:
- Be Specific: They can no longer just say “there was an error.” They must provide a plain-language explanation of the specific error, including the exact line number on your return or schedule that was adjusted.
- Show the Math: The notice must include an itemized computation showing exactly how they arrived at the new numbers.
- Highlight the Deadline: The 60-day abatement deadline (the date by which you must disagree) must now be displayed prominently in bold font on the first page.
- Pilot Certified Mail: The law also initiates a pilot program to send these notices via certified or registered mail, ensuring you actually receive the alert rather than it getting lost in a pile of junk mail.
If your notice looks different or more detailed than in previous years, this is why. Use that extra detail to your advantage — it makes it much easier to verify if the IRS is right or if you should fight back.
The 60-Day Countdown: Your Most Important Right
Math Error Notices come with a strict 60-day deadline that controls your ability to fight the change. This is different from a regular audit, where you usually have longer timelines and more formal appeal steps.
Here is what the 60-day rule means in practice:
- If you request “abatement” within 60 days. You have the right to say, in writing, “We disagree. Please remove this assessment.” When you do that on time, the IRS must reverse the change. If they still believe you are wrong, they then have to use normal audit and deficiency procedures. That process gives you the right to take your case to U.S. Tax Court before you pay.
- If you miss the 60-day window. The IRS change becomes final as a tax assessment. You lose your right to challenge it in Tax Court without paying first. Your remaining option is to pay the tax and then file a claim for refund, which is often slower, more complicated, and more expensive.
The 60 days start from the date printed on the notice, not the day you open the envelope. If the letter sat in a pile of mail for a while, the clock may already be ticking.
How To Respond To A Math Error Notice
Once you understand what changed, your response depends on whether the IRS is correct.
Scenario A: You Agree With the IRS
If you compare the numbers and realize the IRS is right, you generally do not have to formally dispute anything. Here is what to do.
- If you owe money: Pay the balance by the due date listed on the notice to reduce interest and penalties. You can usually pay online, by phone, or by mail using the payment voucher included.
- Update your records: Fix your copy of the tax return so you do not repeat the same error next year. If a preparer made the mistake, share the notice with them so they can adjust their records, too.
You do not need to send a letter just to say “we agree,” unless the notice specifically asks for a signed response.
Scenario B: You Disagree With the IRS
If you believe your original return was correct, or the IRS is working with bad information, you need to act quickly and clearly.
Common examples:
- The notice says your child’s Social Security number is invalid, but it matches the Social Security card you have.
- A credit was removed because the IRS claims a form was missing, but you actually did file it.
- The IRS misread a number on your return or applied the wrong limit.
Here is a practical response plan.
- Call the number on the notice. For simple clerical issues, a phone call can sometimes fix the problem.
- Have the notice and your tax return in front of you.
- Be ready to confirm names, Social Security numbers, filing status, and key line items.
- If not resolved, send a written abatement request. When a phone call is not enough, or the issue is more complex, write to the address listed on the notice.
- Mail it using Certified Mail with Return Receipt so you have proof you responded on time.
- Keep copies of everything you send.
- Be specific and use clear language In your letter, you can say something along these lines: “We are responding to Notice CP11 dated [date of notice]. We request an abatement of the assessment related to the math error identified on this notice.” Then:
- Explain briefly why you believe your original return is correct.
- Attach copies (not originals) of supporting documents, such as a Social Security card, birth certificate, corrected forms, or schedules.
- Include a copy of the notice itself and highlight the lines you are disputing.
If you send that letter within 60 days of the notice date, you preserve your right to move into the regular appeal and Tax Court process if the IRS still disagrees.
When the IRS “Fixes” Your Return, Here Is How You Protect Yourself
A Math Error Notice is not a moral judgment or a full-blown audit. It is a computer saying, “These numbers do not line up with our system, so we changed them.” Sometimes that correction helps you. Sometimes it hurts you. Sometimes it is wrong.
For the 2026 filing season, your best strategy is straightforward:
- File as accurately as possible the first time.
- Double-check Social Security numbers, names, and any credits that need extra forms.
- If a letter shows up, read the code, compare the numbers, and decide quickly whether you agree.
- If you do not agree, use your 60-day right to request abatement and protect your ability to fight the change before paying.
Handled calmly and promptly, a Math Error Notice becomes a small detour, not a financial disaster. The key is simple. Do not ignore the envelope, and do not let the 60 days quietly run out on your rights.
