Home / Notices and letters

IRS Notices CP11 and CP12: Math Error Changes to Your Return

By the Tax Relief Answers editors · Sources checked November 3, 2026 · How we check

The short answer

CP11 and CP12 both mean the IRS corrected one or more mistakes on your tax return. On a CP11 the correction means you owe money or owe more; on a CP12 you are now due a refund or your refund changed. If you disagree, contact the IRS by the date on the notice: the IRS says it will reverse most of these changes when you ask, and the law gives you 60 days after a math error notice is sent to request that abatement (26 U.S.C. 6213(b)(2)). Sources: IRS, Understanding your CP11 notice and IRS, Understanding your CP12 notice, both reviewed by the IRS on November 3, 2026.

CP11 and CP12 side by side

Why the deadline matters

A math error notice is not a notice of deficiency, so it does not by itself open a Tax Court case (26 U.S.C. 6213(b)(1)). Instead, the law lets you ask for an abatement within 60 days after the notice is sent, and the IRS must then abate the assessment. If the IRS still wants the tax, it has to use the normal deficiency process, which ends in a notice like CP3219A with Tax Court rights. The law also bars levy on the assessment while it can still be abated (26 U.S.C. 6213(b)(2)).

The IRS pages say that if you do not contact the IRS by the date on your notice, you lose the formal right to have the change reversed and the right to appeal to the U.S. Tax Court. The IRS adds that it will still consider documents sent later and may reverse the change if it agrees.

Where it sits

CP11 and CP12 come from processing your return, before any collection notice. If a CP11 balance is not paid, the normal collection sequence follows: CP14, then CP501 and CP503, then CP504.

Steps

1. Read the "What we changed on your return" section of the notice.
2. Compare it with your copy of the return. Correct your copy for your records, but do not send it to the IRS.
3. If you agree with a CP11, pay by the date shown. If you cannot pay in full, apply for an IRS payment plan.
4. If you disagree, call the toll-free number in the "Where to find more information" section before the date on the notice. The IRS says the phone is the fastest route and that you can fax missing forms during the call.
5. If you need to change something else on the return, file Form 1040-X, the IRS says.
6. If you cannot pay because of circumstances beyond your control, ask about penalty relief by the due date; see first-time penalty abatement.

Common mistakes

• Mailing a corrected full return. The IRS says not to send your corrected copy.
• Missing the date on the notice. Late information is still considered, but the formal reversal right and the Tax Court route are lost, the IRS says.
• Assuming a reversal ends the matter. If your original return is not supported, the IRS may send the case for audit, and it says the audit staff will contact you within 6 weeks.
• Waiting on a CP11 balance. Interest runs from the return due date if you do not pay in full by the date on the notice.

Questions about a CP11 or CP12 are handled with one free phone call to the number on the notice. A Low Income Taxpayer Clinic can help with a disputed change if you meet its income limits. Not sure the notice is real? See how to check an IRS letter.

FAQ