A Social Security number is a nine-digit identifier the federal government assigns to track your lifetime earnings and determine your eligibility for retirement, disability, and survivor benefits. Congress created it under the Social Security Act of 1935 as an internal bookkeeping tool, but over the decades it has grown into the default personal identifier used for tax filing, credit reporting, banking, and access to most government services. Roughly 450 million numbers have been issued since the program began.
Why the Number Exists
The Social Security Act, now codified at 42 U.S.C. Chapter 7, came out of the Great Depression as a way to fund retirement benefits through payroll taxes.1Office of the Law Revision Counsel. 42 USC Ch. 7 – Social Security The Social Security Administration needed a reliable way to match each worker’s contributions to the right person across a career that might span 40 or 50 years and dozens of employers. The nine-digit number became that link.
That is still its core function. What has changed is everything layered on top: the IRS uses it to identify tax returns, lenders check it when pulling credit reports, banks require it to open accounts, and countless private businesses treat it as proof you are who you say you are. No single law directed this expansion, which is one reason the number has become such a valuable target for identity theft.
What the Nine Digits Mean
An SSN is written as three digits, two digits, four digits, and each block originally had an administrative meaning. The first three digits were the Area Number and identified the office where the card was issued, with lower numbers on the East Coast and higher numbers running west. The middle two digits were the Group Number, which broke each area into smaller filing batches in a deliberately non-sequential order (odd numbers 01 through 09 first, then even numbers 10 through 98) so paper clerks could manage the files. The last four digits, the Serial Number, ran consecutively from 0001 to 9999 within each group.2Social Security Administration. The SSN Numbering Scheme
On June 25, 2011, the SSA scrapped that structure and switched to fully randomized assignment. Two reasons drove the change. Randomization makes numbers much harder to guess (under the old system, knowing someone’s birthplace and rough age narrowed the possible Area Numbers), and it frees up combinations that had never been usable under the geographic system, which extends how long the nine-digit format can last.3Social Security Administration. Social Security Number Randomization Existing numbers were not changed. If yours was assigned before June 2011, it still carries the geographic fingerprint of where you applied.
Some digit combinations are never assigned and immediately flag a fake number. The SSA treats a number as invalid if the first three digits are 000, 666, or anything from 900 through 999; if the middle two digits are 00; or if the last four digits are 0000.4Social Security Administration. Invalid Social Security Numbers (SSNs)
Who Is Eligible and How to Get One
Federal regulations set out three eligible groups: U.S. citizens, lawful permanent residents, and noncitizens with work authorization from the Department of Homeland Security.5eCFR. 20 CFR Part 422 – Organization and Procedures A narrow exception exists for noncitizens without work authorization who need a number for a valid non-work reason, such as a federal law that requires an SSN to receive a specific benefit.
Most people in the U.S. get their number at birth. The Enumeration at Birth program lets parents apply during hospital birth registration, and about 99% of infant SSNs are now assigned this way.6Social Security Administration. Enumeration at Birth Process Adults applying for the first time file Form SS-5 with the SSA and provide at least two original documents (or copies certified by the issuing agency) proving age, identity, and either U.S. citizenship or lawful immigration status. A birth certificate typically covers age and citizenship together, a driver’s license or passport proves identity, and noncitizens must show a current immigration document such as a Permanent Resident Card (Form I-551) or Employment Authorization Document (Form I-766). Photocopies and notarized copies are not accepted, and the SSA verifies every document with the agency that issued it.7Social Security Administration. Learn What Documents You Will Need to Get a Social Security Card
People who are not eligible for an SSN but still owe federal taxes can apply to the IRS for an Individual Taxpayer Identification Number (ITIN). An ITIN is also nine digits, but it exists solely for tax purposes. It does not qualify you for Social Security benefits, the Earned Income Tax Credit, or work authorization, and it does not change your immigration status. If you later become eligible for an SSN, you must notify the IRS so your records are combined under the new number.8Internal Revenue Service. Individual Taxpayer Identification Number (ITIN)
The Three Types of Social Security Cards
The SSA issues three card types, and the legend printed on the face tells employers what the holder can do:9Social Security Administration. Types of Social Security Cards
- An unrestricted card shows only the name and number, with no legend. It goes to U.S. citizens and permanent residents who can work without restriction.
- A card marked “Valid for Work Only With DHS Authorization” goes to people lawfully in the U.S. on a temporary basis who have work permission from the Department of Homeland Security. When that authorization expires, so does the right to work under the card.
- A card marked “Not Valid for Employment” goes to noncitizens without work authorization who need an SSN for a non-work reason, such as receiving a federally mandated benefit.
Employers are legally required to respect those legends. Knowingly accepting a “Not Valid for Employment” card as proof of work eligibility exposes an employer to federal civil penalties.
When You Have to Give Out Your Number
Section 7 of the Privacy Act of 1974 limits how government agencies can demand your SSN. No federal, state, or local agency may deny you a right, benefit, or privilege because you refused to disclose it, unless a federal statute specifically requires the disclosure or the agency’s record system predates January 1, 1975, and required the number by law at that time. When an agency does ask, it must tell you three things: whether disclosure is mandatory or voluntary, the legal authority for the request, and how the number will be used.10Office of the Law Revision Counsel. 5 USC 552a – Records Maintained on Individuals
That protection covers government agencies only. Private companies, including landlords, medical offices, and cell phone carriers, are generally free to ask for your SSN, and you are generally free to decline, though they can refuse to do business with you if you do.
The Penalties for Misuse
Using someone else’s SSN, making up a number, or lying on an SSN application is a federal felony under Section 208 of the Social Security Act. Convictions carry up to five years in federal prison, or up to ten years for offenders in professional roles tied to benefits determinations, such as claimant representatives or healthcare providers submitting medical evidence.11Office of the Law Revision Counsel. 42 USC 408 – Penalties The SSA’s Office of the Inspector General investigates these cases and refers them for prosecution.
Protecting Your Number
Your SSN unlocks credit applications, tax refunds, employment records, and government benefits, which makes it the single most valuable piece of personal data a thief can take. A few concrete steps make misuse much harder.
Credit Freezes and Fraud Alerts
You can freeze your credit at all three major bureaus (Equifax, Experian, and TransUnion) for free, and each bureau must process an online or phone request within one business day.12USAGov. How to Place or Lift a Security Freeze on Your Credit Report A freeze blocks lenders from pulling your report, which stops most fraudulent account openings. You will need to lift it temporarily when you legitimately apply for credit.
Fraud alerts are a lighter option. Contacting any one of the three bureaus places a free one-year alert and automatically triggers notification to the other two.13IdentityTheft.gov. Steps to Take The alert asks creditors to verify your identity before opening new accounts, but it does not block access the way a freeze does.
E-Verify Self-Lock
If you are worried about someone using your SSN to get a job, the E-Verify Self-Lock tool lets you lock the number inside the E-Verify system. When a locked SSN is entered by an employer, the system flags a mismatch.14E-Verify. Self Lock You will need a myE-Verify account and challenge questions, and the lock stays on until you remove it, so remember to unlock it before starting a new job with an E-Verify employer.
If Your Number Is Already Compromised
Start at the Federal Trade Commission’s IdentityTheft.gov. Filing there creates an official Identity Theft Report and generates a personalized recovery plan.13IdentityTheft.gov. Steps to Take If someone filed a tax return under your SSN, submit IRS Form 14039 (Identity Theft Affidavit). If someone used your SSN for work, review your earnings history through your my Social Security account and contact your local SSA office to dispute wages you didn’t earn.
In extreme cases where misuse is ongoing and every other remedy has failed, the SSA may assign a new number, but the bar is high: you must show that someone continues to use your number despite your efforts to stop them. The SSA will not issue a new number simply because a card was lost, to help you avoid bankruptcy consequences, or to help anyone dodge legal obligations.15Social Security Administration. Identity Theft and Your Social Security Number A new number also carries its own cost: your credit history does not follow you, so you would effectively start over with lenders.