Updated October 19, 2026. Quick answer: Letter 12C says the IRS needs more information to process your individual return, such as missing or corrected forms or schedules, verification of income, withholding and credits, or verification of taxpayer identification numbers. Send the information within 20 days, respond even if you disagree, and do not file Form 1040-X. Any refund is sent about 6 to 8 weeks after the response.
Quick reference
| Item | As the IRS describes it |
|---|---|
| What it is | A request for more information to process your return |
| Reply within | 20 days |
| Refund timing | About 6 to 8 weeks after the IRS receives your response |
| If you disagree | Respond anyway with a clear explanation |
| Do not file | Form 1040-X |
| Time to allow for an IRS reply (Tax Topic 651) | At least 30 days |
| Refund after a notice, if you owe nothing else the IRS must collect (Tax Topic 651) | Within 6 weeks of the notice date, which is 42 days |
What the letter says
“We need more information to process your individual income tax return.” The IRS lists what it may need: missing or corrected forms or schedules supporting your Form 1040 or 1040-SR entries, verification of income, withholding and credit amounts, and verification of taxpayer identification numbers or Social Security numbers.
What you need to do
“Read your letter carefully.” Then “Send the information we requested within 20 days.”
If you disagree: “Respond to the letter, even if you disagree, providing a clear explanation of what you disagree with.”
Why not a Form 1040-X
“Do not file a Form 1040-X, Amended U.S. Individual Income Tax Return.” Letter 12C asks for items that complete the return you already filed, so an amended return is the wrong tool. A 1040-X is for correcting a return after it is processed; see Form 1040-X for when that applies.
What happens after you reply
The IRS sends any refund due about 6 to 8 weeks after it receives your response.
Letter 12C is not a math-error notice: those are CP12, CP11 and CP13. Check that yours is real with is this IRS letter real.
Where this comes from
The IRS page read for this page is the one titled Understanding your letter 12C on irs.gov, as served on 2026-10-19. Your own notice controls: the dates, amounts and phone or fax numbers on it are the ones to use, and the IRS can update its page after we read it.
For IRS notices in general, Tax Topic 651 says “You should allow at least 30 days for a reply from the IRS.” It also says “Keep a copy of all IRS correspondence for your tax records.” On refunds it says “you’ll receive the refund within 6 weeks of the notice date.”
Common mistakes
- Missing the 20 days because you plan to send everything at once. Send what the letter asks for by the date.
- Filing Form 1040-X in reply. The letter says not to.
- Not responding because you disagree. The IRS says respond anyway and explain.
- Sending a different document than the letter asks for.
Related: how to respond to an IRS notice, is this IRS letter real, and IRS notices and letters.
Sources
- IRS, Understanding your Letter 12C (irs.gov): https://www.irs.gov/individuals/understanding-your-letter-12c
- IRS, Tax Topic 651, Notices: what to do (irs.gov): https://www.irs.gov/taxtopics/tc651
Read from the IRS pages linked above on October 19, 2026. General information, not tax or legal advice. Your own notice and facts decide the outcome, and the number printed on your notice is the right place to confirm anything consequential.