On the paper
Find the line the IRS changed, the figure it used, and the figure you reported.
The notice sets out the changed lines side by side. Read that comparison rather than the balance, because the difference between the two columns names the item in dispute, and it is often a credit the IRS recomputed rather than arithmetic you got wrong.
Find the date printed on your notice or letter. The paper notice and the current official IRS instructions control, even where this page differs.
Where this notice comes from
The Internal Revenue Code lets the IRS correct certain mathematical and clerical errors without issuing a notice of deficiency first. That summary route is why a correction can appear as a bill with no examination behind it.
The tradeoff is the objection window on the notice. A timely objection generally forces the IRS out of the summary route and into the ordinary deficiency procedure, which is the procedure that leads to the Tax Court.
Most CP11 corrections trace to a credit computed differently, a dependent claimed on two returns, or an entry the IRS read from a different line than the one intended.
A dependent claimed on two returns is worth singling out, because it is the one CP11 cause that involves another person's filing. The IRS does not tell either filer who the other is, and the correction goes to whichever return it processed second.
General next steps
- Put the notice next to your return and identify every line that changed.
- If the correction is right, follow the payment instructions on the notice.
- If it is wrong, contact the IRS by the date shown on the notice, because that date protects rights the notice describes.
Timing
The IRS page for CP11, checked September 10, 2026, gives no general period and refers to the date shown on the notice. That page also states that missing the date for disagreeing costs the formal right to have the change reversed and the right to appeal the decision to the United States Tax Court. The date printed on the notice controls; do not calculate a deadline from this page.
What CP11 does not mean
A math error correction is not an audit. The IRS did not examine the return or ask for records. It also does not mean the IRS is right; the math error authority is summary in nature, which is exactly why the notice carries a window to object.
What this page cannot tell you
This page cannot verify the figures in your return or tell you whether the IRS correction is right.
Official sources
If anything on this page differs from your notice or from current IRS instructions, follow the notice and the IRS.
Common questions
What do I lose if I miss the date on the notice?
The IRS page for CP11 states that you lose the formal right to have the change reversed and the right to appeal the decision to the United States Tax Court. That is why the objection date matters more than the payment date.
Do I need to file an amended return to disagree?
The notice asks you to contact the IRS by the date shown. An amended return is a separate filing that follows its own processing path and does not preserve the objection window.
Is the balance on a CP11 subject to penalties?
Interest and the failure-to-pay penalty run on an unpaid balance from the date the tax was due, not from the date of the notice. That is why a correction made months after filing can arrive with additions already attached.
Can I pay the balance and still dispute it?
Paying stops the failure-to-pay penalty and interest from growing on the amount paid. It does not by itself resolve the disagreement, which still runs through the response the notice describes.