The notice number on an IRS letter about your tax return is printed in the upper right corner of the first page. It starts with either CP or LTR, followed by a string of digits (for example, CP2000 or LTR 4464C). That code identifies exactly which type of letter you’re holding, what the IRS is asking about, and how long you have to respond. It is the first thing to write down before you do anything else with the letter.
Where the Number Appears on the Page
Look at the top right of page one, near the date the notice was issued and the tax year it concerns. CP numbers are usually two to four digits. LTR numbers tend to be longer and often end with a letter, like 4464C or 3219.
Your Social Security number or taxpayer identification number also appears on the notice, sometimes close to the notice number. Don’t confuse them. The notice number identifies the letter; the SSN identifies you. If the correspondence has no CP or LTR code anywhere on it, treat that as a warning sign rather than an oversight, because every piece of genuine IRS correspondence carries one.1Internal Revenue Service. Understanding Your IRS Notice or Letter
You can also see many of your notices through your IRS Online Account, which stores copies and lets you go paperless for certain notice types.2Internal Revenue Service. Online Account for Individuals If a letter arrived at your house and you want to confirm the IRS actually sent it, logging into your account is one of the fastest checks available.
What CP and LTR Mean
The two-letter prefix tells you how the correspondence was generated.
CP stands for “Computer Paragraph.” An automated IRS system flagged something on your account and generated the notice without a specific employee starting the file. Most balance-due notices, math-error notices, and information-mismatch notices carry a CP number.
LTR notices are letters, often tied to a more individualized review such as an identity verification request, an audit, or a formal notice of deficiency. LTR correspondence more often requires you to send documentation or take a specific action rather than simply review a proposed change.
Neither prefix is inherently more serious than the other. What matters is the specific number that follows.
Looking Up What Your Number Means
The IRS keeps a searchable index of every CP and LTR number. Type the number into the IRS website and you’ll get a plain-language explanation of what the notice covers, why it was sent, and what your next steps should be.1Internal Revenue Service. Understanding Your IRS Notice or Letter A few come up often enough that recognizing them by number is useful:
- CP2000. The IRS received income information from an employer, bank, or other third party that doesn’t match what you reported on your return. This is a proposal to adjust your return, not a bill, and you have 30 days to respond (60 days if you’re outside the country).3Internal Revenue Service. Topic No. 652, Notice of Underreported Income – CP2000
- CP504. A formal notice of intent to levy. It arrives only after earlier balance-due notices went unanswered, and it warns that the IRS will begin seizing your state tax refund or other property if you don’t pay within 30 days.4Internal Revenue Service. Notice CP504
- Letter 3219. The statutory notice of deficiency, also called the 90-day letter. It gives you exactly 90 days (150 days if you’re outside the United States) to file a petition with the U.S. Tax Court challenging the proposed additional tax. Missing that 90-day window closes the Tax Court route permanently; the only remaining option is to pay in full, file a refund claim, and then sue in federal district court or the Court of Federal Claims. Direct negotiation with the IRS does not pause the clock.5Office of the Law Revision Counsel. 26 USC 6213 – Restrictions Applicable to Deficiencies; Petition to Tax Court
- LTR 4464C. A request to verify your identity before the IRS releases a refund.
The response deadline is always printed on the notice itself. Read it before anything else you do with the letter.
Confirming the Notice Is Real
Fake IRS letters circulate, and some are polished enough to fool people who have never received real correspondence from the agency. The notice number is one of the first things to check, but not the only one:
- Genuine notices always carry a CP or LTR number in the upper right corner. A letter without one is suspect.
- The IRS never demands immediate payment by gift card, wire transfer, or prepaid debit card. Real notices give you a defined response window and point you to official payment channels.6Internal Revenue Service. How to Know It’s the IRS
- The IRS does not threaten arrest or deportation. Any letter or call using those threats is a scam.
- Legitimate notices reference a specific tax year and explain the reason for the contact. Vague warnings about “your tax account” with no details are a red flag.
If you’re still not sure, look up the number on the IRS website, call 800-829-1040, or log into your IRS Online Account to see whether the notice appears on your record.2Internal Revenue Service. Online Account for Individuals
Why the Number Matters When You Respond
Federal law requires the IRS to describe the basis for any tax, interest, or penalty in every notice it sends.7Office of the Law Revision Counsel. 26 USC 7522 – Content of Tax Due, Deficiency, and Other Notices The notice number is how the IRS organizes that obligation internally. Each code corresponds to a specific procedure and a specific team.
When you call the IRS or write back, referencing the notice number routes your communication to the people already working the case. Without it, the representative has to search your entire account history to figure out which issue you’re asking about. Include the notice number, the notice date, the tax year, and your taxpayer identification number in any written response, and send it to the address printed on the notice rather than to a general IRS office. If a return envelope came with the letter, use it. Some notices now include an access code for the IRS Document Upload Tool, which lets you send documents digitally and get immediate confirmation of receipt.8Internal Revenue Service. IRS Document Upload Tool
Keep the Notice After the Issue Is Resolved
Store every IRS letter you receive, even after the matter is closed. If the IRS later claims you didn’t respond, or disputes a timeline, the notice is your evidence of when you were contacted and what was proposed. A simple log of the notice number, the date, the tax year, and the response deadline is enough. Track them as you get them. People who keep this record resolve tax issues faster than people who stuff notices in a drawer.