What is SSN (Social Security Number)?

How it works

The SSN is issued by the Social Security Administration (SSA), not the IRS. Format: XXX-XX-XXXX, nine digits, never reissued. Once assigned, the number is permanent and personal.

The SSN serves three overlapping functions:

1. Tax identification. The IRS uses the SSN as the default TIN for US persons on every federal filing — Form 1040, W-2, 1099, W-9, payroll deposits, retirement plan filings.
2. Social Security and Medicare. The SSA tracks earnings under the SSN to compute future retirement and disability benefits, plus Medicare eligibility at age 65.
3. De-facto national ID. Banks, employers, credit reports, healthcare providers, state DMVs, and countless private services use the SSN as a unique personal identifier — even though that was never its statutory purpose.

Who can get an SSN

The SSA issues SSNs only to:

• US citizens (at birth or via naturalisation).
• Lawful permanent residents (green card holders).
• Non-immigrants with US work authorization — H-1B, L-1, O-1, certain F-1 / J-1 categories with employment authorization documents (EAD), TN visa holders, etc.

A B-1 / B-2 tourist or business visitor cannot get an SSN. Neither can a non-resident with no immigration status who is doing business with US clients from abroad.

What non-residents use instead

For individuals: ITIN (Individual Taxpayer Identification Number), issued by the IRS via Form W-7. Functions as the SSN-equivalent for federal tax filings only — does not authorize work, does not provide Social Security benefits, does not act as an ID for non-tax purposes.

For entities: EIN (Employer Identification Number), issued by the IRS via Form SS-4. Works for LLCs, corporations, partnerships, trusts. Foreign owners can apply by fax without any SSN.

Examples

• Indian software engineer on H-1B in San Francisco. Has US work authorization → gets an SSN within weeks of starting employment. Files Form 1040 (resident under SPT) using the SSN; the SSN tracks his Social Security earnings for future benefits.
• French founder running a Wyoming LLC from Paris. No US presence, no work authorization → no SSN. The LLC has an EIN (applied for via Form SS-4 by fax). If treaty rates need to be claimed on a W-8BEN, he applies for an ITIN separately via Form W-7.

Common mistakes

• Assuming you need an SSN to do anything in the US. You don't, for business setup. EIN is enough for the LLC, ITIN is enough for personal tax filings; no SSN needed.
• Confusing SSN with ITIN on Form W-9. Foreign-owned single-member LLCs sometimes mistakenly try to file W-9 in the LLC's name with an EIN. The right form for a foreign-owned disregarded LLC is W-8BEN in the owner's name — not W-9.
• Sharing SSN unnecessarily. US identity-theft scams target SSNs aggressively. Use the EIN for any business-context request that doesn't legally require the personal SSN.
• Believing SSN expiration. SSNs don't expire, ever — unlike ITINs (3-year inactivity) and certain EINs (none expire, but inactive ones can be hard to verify).