How Are Social Security Numbers Formatted: 3 Segments and Rules

A Social Security Number is formatted as nine digits split into three groups by two hyphens: three digits, then two digits, then four digits, written as 123-45-6789. The Social Security Administration has used this same nine-digit pattern since 1936.¹ What has changed is how the agency picks the digits: numbers issued on or after June 25, 2011 are assigned randomly, while numbers issued before that date encoded information about where the card was applied for.²

The Three Named Segments

The SSA gives each part of the number a name. The first three digits are the Area Number. The middle two are the Group Number. The last four are the Serial Number. So the full pattern is AAA-GG-SSSS.¹ Those labels come from the original assignment system and still appear in SSA documentation, even though two of the three no longer carry their historical meaning.

What Each Segment Used to Mean

Before June 2011, each piece of your number told you something about when and where it was issued.

The Area Number reflected geography. Before 1972, it corresponded to the Social Security office that processed the application. After 1972, when the SSA centralized card issuance in Baltimore, the area number was based on the ZIP code in the applicant’s mailing address. Numbers generally ran lower on the East Coast and higher toward the West.
The Group Number handled internal administrative processing. It was not assigned in consecutive order; the SSA followed a staggered pattern to break area numbers into smaller blocks.
The Serial Number ran consecutively from 0001 through 9999 within each group.

All three of these rules come from the SSA’s original numbering scheme.¹ If your SSN was issued before June 25, 2011, the area number still reflects where you or your parents applied for the card. Numbers issued after that date follow completely different rules.

What Randomization Changed in 2011

On June 25, 2011, the SSA switched to randomized assignment. The area number no longer corresponds to a state or region, the group number no longer follows the staggered administrative pattern, and the serial number is no longer sequential within a group.² The nine-digit, three-hyphen format itself stayed the same. Only the method of choosing the digits changed.

Two reasons drove the switch. Because the old system tied area numbers to geography, anyone who knew where and roughly when you were born could narrow down your SSN significantly. Randomization made that kind of reconstruction much harder. The geographic allocation system was also running out of numbers in some high-population states while other states still had large unused blocks. Randomizing across the full pool extended the life of the nine-digit format nationwide.³

Digit Combinations That Are Never Valid

Not every nine-digit sequence is a legitimate SSN. The SSA permanently excludes several patterns, and these exclusions survived the switch to randomization:

Area number 000. No SSN begins with three zeros.
Area number 666. Permanently excluded.
Area numbers 900 through 999. The entire 900 range is reserved and never assigned as an SSN.
Group number 00. The middle two digits are never 00.
Serial number 0000. The last four digits are never all zeros.

These rules are documented in the SSA’s Program Operations Manual.⁴ The randomization FAQ confirms they carry forward under the new system.³

The 900 through 999 exclusion matters in one practical way. The IRS uses the same nine-digit, three-hyphen format for Individual Taxpayer Identification Numbers, but every ITIN starts with the digit 9, with the fourth and fifth digits falling within specific ranges (50–65, 70–88, 90–92, or 94–99).⁵ A nine-digit tax ID starting with 9 is an ITIN, not an SSN.

The Same Format Appears on Three Card Types

The number format is identical on every Social Security card. What varies is the legend printed under the number, which signals the holder’s work authorization:

An unrestricted card shows the name and SSN with no legend. It is issued to U.S. citizens and lawful permanent residents and allows work without restriction.
A card printed with “VALID FOR WORK ONLY WITH DHS AUTHORIZATION” is issued to people admitted to the U.S. temporarily who have work authorization from the Department of Homeland Security.
A card printed with “NOT VALID FOR EMPLOYMENT” is issued to people who need an SSN for a non-work reason, such as receiving a federal benefit that requires one, but who do not have DHS work authorization.

All three card types display the same nine-digit AAA-GG-SSSS format.⁶ The distinction lives entirely in the legend, not the number itself.

Reading Your Own Number

If your SSN was issued before June 25, 2011, the first three digits tell you the general region where the card was applied for, following the ZIP-code-based scheme in place from 1972 onward, or the issuing office’s location before that. The middle two digits reflect the SSA’s staggered administrative sequence rather than a running count, so a lower group number does not mean an older card. The last four digits were assigned in order within each group.

If your SSN was issued on or after that date, none of the digits carry geographic or sequential meaning. The nine digits still fall into the same three named segments, and the same exclusions (000, 666, 900–999, group 00, serial 0000) still apply, but the specific numbers were drawn from the SSA’s randomized pool.²

1
Social Security Administration. The SSN Numbering Scheme
2
Social Security Administration. Social Security Number Randomization
3
Social Security Administration. Social Security Number Randomization Frequently Asked Questions
4
Social Security Administration – Program Operations Manual System (POMS). Invalid Social Security Numbers (SSNs)
5
Internal Revenue Service. IRS Individual Taxpayer Identification Number (ITIN) Real-Time System
6
Social Security Administration. Types of Social Security Cards