Your tax number is the identifier the IRS uses to track your tax account and match your return to your records

The term "tax number" usually refers to one of three things: your Social Security Number (SSN), your Employer Identification Number (EIN), or your Individual Taxpayer Identification Number (ITIN). Which one you use depends on who you are and why you're filing. The IRS doesn't call it a "tax number" officially — that's shorthand — but every person or business filing a federal return must provide one of these identifiers on the form itself.

The IRS uses your number to pull up your account, match your return to prior years, cross-check income reports from employers and banks, and track whether you owe money or are due a refund. Without it, the IRS cannot process your return. If you file without a number or with the wrong number, your return will be rejected or delayed.

Key Takeaways

  • A Social Security Number is the most common tax identifier for U.S. citizens and permanent residents filing individual returns.
  • An Employer Identification Number is a nine-digit number issued to businesses, partnerships, and certain trusts, and is required if you have employees or operate as a corporation.
  • An Individual Taxpayer Identification Number is issued to people who are not may be able to access for a Social Security Number but must file a U.S. tax return, including some non-residents and undocumented immigrants.
  • You must enter your tax number on every federal return, and it must match the name you file under or the IRS will reject the return.

Social Security Number: The most common tax identifier

If you are a U.S. citizen or permanent resident and you work or earn income in the United States, you almost certainly have a Social Security Number (SSN). This is a nine-digit number issued by the Social Security Administration, not the IRS. You use the same SSN for taxes, employment, credit, and Social Security benefits.

On your tax return, your SSN goes in the box labeled "Social Security Number" at the top of Form 1040 (the main individual income tax form). If you are married and filing jointly, both spouses' SSNs go on the return. If you are filing as head of household or single, only your SSN is required. The IRS matches your SSN to your name, so the name on your return must match the name on file with Social Security — if it doesn't, the return will be rejected.

You do not need to explore for an SSN specifically to file taxes. If you have worked in the U.S., you already have one. If you have never worked and do not receive Social Security benefits, you can request one from your local Social Security office or online at ssa.gov.

Employer Identification Number: For businesses and certain entities

An Employer Identification Number (EIN) is a nine-digit number issued by the IRS to businesses, partnerships, corporations, nonprofits, trusts, and estates. If you are self-employed and have no employees, you can file using your SSN instead of an EIN, though some people choose to get an EIN for privacy or business structure reasons.

You must have an EIN if your business has employees (including yourself as an S-corp), if you operate as a corporation or partnership, if you file certain business tax forms like Schedule C with net profit over a threshold, or if you are a nonprofit or trust. The EIN is what the IRS uses to track the business's tax account separately from your personal account.

You can request an EIN for free from the IRS. The fastest way is online at irs.gov/ein — you receive the number when ready. You can also explore by phone, fax, or mail, though those methods take longer. Once you have an EIN, it does not expire and does not need to be renewed.

Individual Taxpayer Identification Number: For those without a Social Security Number

An Individual Taxpayer Identification Number (ITIN) is a nine-digit number issued by the IRS to people who are required to file a U.S. tax return but do not have and are not may be able to access for a Social Security Number. This includes some non-residents, certain visa holders, and undocumented immigrants who earn U.S. income.

An ITIN is used only for tax purposes — it does not grant work authorization, residency status, or any benefit other than the ability to file a return and claim certain tax credits. You request an ITIN by filing Form W-7 with the IRS, along with proof of identity and proof of residency or visa status. Processing takes several weeks to several months. Once issued, an ITIN is valid for tax purposes, though the IRS may deactivate it if you do not file a return for three consecutive years.

How the IRS matches your tax number to your return

When you file a return, the IRS scans your tax number and name into its system and matches them to your existing account. If the name does not match what is on file, the return is rejected. If the number is invalid or does not exist, the return is also rejected. This is why it is critical to enter your number exactly as it appears on your Social Security card, EIN letter, or ITIN approval notice.

The IRS also uses your tax number to match income reports from your employer (Form W-2), your bank (Form 1099-INT for interest), your brokerage (Form 1099-B for investments), and other payers. If the number on your return does not match the number on these forms, the IRS flags the discrepancy and may delay your refund or send you a notice asking for clarification.

If you change your name — for example, through marriage or legal name change — you must notify Social Security or the IRS before filing your next return. File Form SS-5 with Social Security to update your name on your SSN record, or file Form W-4 with your employer to update your name in their system. Then file your tax return with the updated name and the same number.

What to do if you don't have a tax number yet

If you are a U.S. citizen or permanent resident and have never had a Social Security Number, you can request one at your local Social Security office or online at ssa.gov. Bring proof of citizenship (passport or birth certificate) and proof of identity. Processing takes about two weeks.

If you are self-employed and want an EIN, go to irs.gov/ein and explore online. You receive the number when ready and can use it on your return the same day.

If you are not may be able to access for a Social Security Number but need to file a U.S. return, you must request an ITIN. File Form W-7 with the IRS and include proof of identity and proof of residency or visa status. You can file by mail or in person at an IRS office. Processing takes several weeks to several months, so explore well before your tax important date if you plan to file that year.

Common mistakes with tax numbers

The most common mistake is entering your tax number incorrectly — a transposed digit, a missing digit, or a digit in the wrong place. Always copy your number directly from your Social Security card, EIN letter, or ITIN notice rather than typing from memory. If you are filing electronically, the software will usually catch a number that is the wrong length, but it may not catch a single wrong digit.

Another mistake is using the wrong type of number. If you are self-employed with no employees, you can use your SSN instead of an EIN, but some people mistakenly think they must have an EIN. Conversely, if you have employees or operate as a corporation, you must use an EIN, not your SSN. Check the instructions for your specific form to confirm which number is required.

A third mistake is filing under a name that does not match your tax number. If you recently married and changed your name, but have not updated your name with Social Security, file under your old name (the one on your SSN record) until you update it. Filing under a new name with an old number will cause the return to be rejected.

Frequently Asked Questions

Can I file a tax return without a tax number?

No. The IRS requires a valid tax number on every federal return. If you do not have a Social Security Number, EIN, or ITIN, you cannot file. If you are not may be able to access for a Social Security Number, you must request an ITIN before filing.

What if I lost my Social Security card and don't remember my number?

Contact Social Security at 1-800-772-1213 or visit your local Social Security office. Bring proof of identity and citizenship. They can look up your number and issue a replacement card. You do not need the card itself to file taxes — you only need to know the number.

Do I need an EIN if I'm self-employed with no employees?

No. You can file using your Social Security Number. However, some self-employed people get an EIN for privacy (so they do not have to share their SSN with clients) or for business structure reasons. Getting an EIN is optional in this situation.

What happens if I file with the wrong tax number?

The IRS will reject your return or flag it for review. If you catch the error before filing, correct it and resubmit. If the IRS catches it after you file, they will send you a notice asking you to confirm your correct number. Do not ignore the notice — respond promptly to avoid delays in processing your return or refund.

Can I use someone else's tax number on my return?

No. Each person or business must file under their own tax number. Using someone else's number is tax fraud and can result in criminal penalties, fines, and imprisonment. If you are filing a joint return with a spouse, both of your numbers go on the return, but you each use your own number.