Every taxpayer in the US needs a tax identification number — it’s how the IRS tracks what you earn and what you owe. Which type you need depends on who you are and what you’re filing for. Here’s the full breakdown.
Types of Tax ID Numbers
| TIN Type | Issued By | Who It’s For | Format |
|---|---|---|---|
| Social Security Number (SSN) | Social Security Administration | US citizens, permanent residents, eligible non-citizens | XXX-XX-XXXX |
| Employer Identification Number (EIN) | IRS | Businesses, trusts, estates, nonprofits | XX-XXXXXXX |
| Individual Taxpayer Identification Number (ITIN) | IRS | Non-residents and resident aliens without SSN eligibility | 9XX-XX-XXXX (starts with 9) |
| Adoption Taxpayer Identification Number (ATIN) | IRS | Adoptive parents pending adoption finalization | Same format as SSN |
| Preparer Tax Identification Number (PTIN) | IRS | Tax professionals who prepare returns for compensation | P-XXXXXXXX |
Social Security Number (SSN)
The SSN is the primary taxpayer ID for individuals in the US. It’s issued at birth for US citizens and can be obtained by eligible non-citizens working in the US.
You use your SSN for:
- Personal federal and state tax returns (Form 1040)
- W-2 employment (employer matches SSN to your wages)
- W-9 requests from clients if you’re a sole proprietor
- Investment accounts and bank accounts
- Claiming dependents (each dependent also needs a valid SSN)
Protecting your SSN: Your SSN should be shared only when legally required — tax returns, employer records, financial institutions. Never carry your Social Security card in your wallet. The IRS communicates by mail, not phone or email.
Employer Identification Number (EIN)
An EIN is the business equivalent of an SSN. It’s required for:
| Situation | EIN Required? |
|---|---|
| Partnership or multi-member LLC | Yes |
| Corporation (C or S) | Yes |
| Sole proprietor with employees | Yes |
| Sole proprietor with no employees | Optional but recommended |
| Trust or estate | Yes |
| Nonprofit organization | Yes |
| Keogh retirement plan | Yes |
How to Apply for an EIN
Online (fastest): irs.gov/ein — free, takes 15 minutes, EIN issued immediately. Available Monday–Friday, 7 a.m. to 10 p.m. Eastern. You cannot apply online if your principal business is outside the US.
By fax: Mail Form SS-4 to the IRS fax line. Receive EIN in about 4 business days.
By mail: Send Form SS-4 to the appropriate IRS address. Allow 4–5 weeks.
Key information needed on Form SS-4:
- Legal name of entity
- Trade name (if different)
- Responsible party’s name and SSN
- Principal business address
- Type of entity
- Reason for applying
- Principal activity of the business
Individual Taxpayer Identification Number (ITIN)
An ITIN allows non-US persons to comply with US tax law. ITINs are for:
- Non-resident aliens with US-source income (dividends, rental income, etc.)
- Resident aliens not eligible for SSN
- Spouses and dependents of US citizens/resident aliens who can’t get an SSN
ITINs do not:
- Authorize work in the US
- Provide Social Security benefits eligibility
- Make you eligible for the Earned Income Tax Credit
Applying for an ITIN
Submit Form W-7 (Application for IRS Individual Taxpayer Identification Number) with:
- Completed W-7 form
- Federal tax return (unless you qualify for an exception)
- Original identity documents OR certified copies from the issuing agency
Three ways to apply:
- Mail to the IRS ITIN Operations address
- In person at an IRS Taxpayer Assistance Center (TAC)
- Through a Certified Acceptance Agent (CAA) — can certify documents so you don’t have to mail originals
ITIN expiration: ITINs not used on a federal tax return for 3 consecutive years expire. ITINs with middle digits 70–88, 90–99 also have required renewal periods. Check irs.gov for current expiration schedules.
When Sole Proprietors Should Get an EIN
Even if not legally required, many sole proprietors get an EIN to:
- Avoid sharing SSN on W-9s — you give clients your EIN instead
- Open a business bank account — most banks require an EIN for business accounts
- Establish business credit — separate EIN supports building a business credit profile
- Add employees in the future — already have the EIN ready
The EIN is free and takes 15 minutes online. There’s no downside to having one.
Your taxpayer identification number is required on Form W-9, which you submit to clients and payers to avoid backup withholding at the 24% rate. For businesses and self-employed individuals who need an Employer Identification Number (EIN) rather than an SSN, see EIN: business tax ID number for how to apply. Your TIN is also required to e-file your annual return — see how to file taxes free for software that handles TIN verification automatically.
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