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SSN

General

US Social Security Number

A 9-digit identifier issued by the US Social Security Administration to track earnings and benefit eligibility, now used broadly as a de facto national ID for tax, credit, and employment purposes.

Definition

A Social Security Number (SSN) is a 9-digit identifier issued by the US Social Security Administration (SSA), originally created to track workers' earnings and eligibility for Social Security benefits. Over time it has become the closest thing the United States has to a national identification number, used for tax filing, employment eligibility verification, credit applications, and identity checks across both government and private sectors.

An SSN is structured as three segments — area number (3 digits), group number (2 digits), and serial number (4 digits) — displayed as AAA-GG-SSSS. Since businesses use a structurally similar but functionally distinct number, the EIN, for their own tax identification.

Formula

An SSN has no checksum digit — there is no mathematical formula that validates whether a given 9-digit sequence is a genuine, error-free number the way Aadhaar's Verhoeff algorithm or a credit card's Luhn check does. Instead, validation relies on structural exclusion rules set by the SSA:

  • The area number (first 3 digits) cannot be 000, 666, or fall in the range 900–999.
  • The group number (middle 2 digits) cannot be 00.
  • The serial number (last 4 digits) cannot be 0000.

A number that passes these structural checks is plausible, not verified — actual issuance can only be confirmed against SSA records, which validator tools don't have access to.

Worked Example

A structurally valid but entirely fictional example: 078-05-1120, displayed with dashes separating the area, group, and serial segments.

Key Things to Know

  • No checksum: SSNs cannot be mathematically verified the way Aadhaar or credit card numbers can — only structural plausibility can be checked, via the SSN Validator.
  • Post-2011 randomization: since June 2011, the SSA assigns area numbers randomly rather than by geography, so the first three digits no longer indicate the holder's state of origin.
  • Reserved ranges are excluded: 000, 666, and 900–999 area numbers have never been issued and always indicate an invalid SSN.
  • Distinct from EIN: an SSN identifies an individual; an EIN identifies a business. Sole proprietors may use either for certain tax filings.
  • Sensitive data: because SSNs are used so widely for identity verification, exposure enables identity theft — always mask or encrypt stored values.
  • Related to Medicare: the Medicare ID (MBI) was introduced specifically to remove SSNs from Medicare cards after decades of SSN-based Health Insurance Claim Numbers.

Related Terms

EINUS Employer Identification NumberMedicare IDMedicare Beneficiary Identifier (MBI)

Frequently Asked Questions

No. Unlike Aadhaar or a credit card number, an SSN has no built-in check digit or checksum formula. Validation instead relies on structural rules — correct length, no all-zero segments, and exclusion of ranges the Social Security Administration has declared invalid or reserved. You can check a number's structural validity with the [SSN Validator](/validators/ssn-validator/).
An SSN identifies an individual for Social Security benefits, tax filing, and employment eligibility, while an [EIN](/glossary/ein/) identifies a business entity for federal tax purposes. Sole proprietors sometimes use their SSN in place of an EIN, but the two numbers are issued by different processes and serve different legal purposes.
No. The area number (first three digits) cannot be 000, 666, or any number from 900–999, and the group number (middle two digits) and serial number (last four digits) cannot both be 00 or 0000. These exclusions exist because the Social Security Administration reserved those ranges and never issued numbers from them.
Before June 2011, the first three digits of an SSN indicated the state where the number was originally issued. Since the SSA's 2011 randomization initiative, area numbers are assigned randomly across the country to extend the usable number pool and reduce the risk of number exhaustion in high-population states.
Formatting for display (XXX-XX-XXXX) is standard and doesn't affect the underlying value, so it's safe to use a tool like the [SSN Formatter](/formatters/ssn-formatter/) for presentation. The actual security concern is exposure and storage of the raw digits, which should always be encrypted or masked regardless of display format.