How to Respond to an IRS Tax Math Error

Receiving a notice from the Internal Revenue Service (IRS) is not always a signal of a serious issue. An IRS math error notice is a common communication indicating the agency’s automated systems found a discrepancy on your tax return. These are computational or data entry mistakes caught during initial processing and are distinct from a formal audit, as they represent an automated correction of a perceived error.

Understanding the IRS Math Error Notice

When the IRS identifies a potential math or clerical error on a tax return, it issues a notice to inform the taxpayer of the adjustment. These notices explain the change and its impact on your tax liability or refund. The most common forms are the CP11, which indicates a balance due, and the CP12, which is sent when adjustments lead to a change in your refund amount. A less frequent notice, the CP13, confirms a miscalculation was corrected but resulted in no change to your tax balance.

Each notice provides specific details about the correction. You will find the notice number and issue date in the top right corner. The body of the letter summarizes the adjustment and includes a line-by-line calculation showing your original figures, the IRS’s corrected figures, and the difference, identifying the specific line item on your tax return that was adjusted.

The notice will also state the new tax calculation, showing either a new balance owed or a revised refund amount. A key piece of information is the response deadline. Under Internal Revenue Code Section 6213, taxpayers have 60 days from the date of the notice to request a reversal of the assessment if they disagree. Failing to respond within this timeframe means the adjustment becomes final, and the IRS may proceed with collection actions if a balance is due.

Common Reasons for a Math Error Notice

The term “math error” is broad and covers more than simple arithmetic mistakes. One of the most frequent triggers is an incorrect calculation of the final tax liability, which can happen when a taxpayer uses the wrong column or line in the tax tables. Simple addition or subtraction errors on any part of the return can also lead to a notice.

Mistakes related to deductions and credits are another primary cause. This can include errors in calculating the standard deduction, especially for taxpayers who are blind or over age 65, as they are entitled to a higher amount. Similarly, miscalculating tax credits, such as the Child Tax Credit or the Earned Income Tax Credit, will often trigger an automated correction, as the rules for eligibility and phase-out amounts are complex.

Data entry mistakes are also a significant source of these notices, as entering a number on the wrong line can alter the tax outcome. A common issue is a discrepancy with a dependent, where a taxpayer claims a dependent who has already been claimed or who does not meet eligibility tests. The IRS cross-references Social Security numbers, and a mismatch will result in an automatic adjustment.

Information Needed to Address the Notice

Before you can effectively respond to an IRS math error notice, it is important to gather all relevant documents to understand the agency’s changes and determine if they are correct. You will need the IRS notice itself and a complete copy of the tax return it references. Compare the figures on your filed return line-by-line against the changes detailed in the IRS notice.

To validate the numbers on your return, you must also collect all supporting documents used for its preparation, such as:

• W-2s from employers
• 1099 forms for self-employment or interest income
• Form 1098 for mortgage interest paid
• Receipts for charitable donations, medical expenses, or records supporting business expenses

If you disagree with the IRS’s changes, these documents will support your original position.

How to Respond to the IRS

After reviewing the notice and your tax records, you must decide whether you agree or disagree with the assessment. The notice will provide instructions for how to proceed.

If you agree with the changes and a balance is due, the notice will include a payment voucher. You can pay the amount owed by mailing a check with the voucher or use an IRS online payment option. If you cannot pay the full amount, you may be able to apply for a payment plan. If the notice indicates a change in your refund and you agree, no further action is needed.

If you disagree with the changes, you must formally notify the IRS within the 60-day window. Write a letter explaining why you believe their adjustment is incorrect. This letter must include your name, Social Security number, the tax year in question, and the notice number.

In your letter, clearly state which adjustments you dispute and provide a concise explanation for your position. You must include copies—not the originals—of the supporting documents that prove your original return was accurate. Mail your complete response package to the address provided on the IRS notice.