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How are Social Security numbers assigned?

Social Security numbers, also known as SSNs, are allocated through a process called randomization that was introduced in June 2011. This system retains the long-standing nine-digit format but assigns a number to each new Social Security cardholder randomly, eliminating methods that date to the inception of Social Security in the mid-1930s.

Those XXX-XX-XXXX numbers allow the Social Security Administration to track individual workers’ lifetime earnings, data it uses to calculate and pay benefits.

Before the 2011 switch, the first three digits were an area number with codes assigned to different states. The two-digit group number divided numbers into blocks within those geographic areas, and the four-digit serial number individualized each full number within that block.

The original system reflected the record-keeping needs of a bygone era when Social Security data was organized for storage in filing cabinets. Besides being better suited to the digital age, randomization serves two main purposes:

  1. It makes life harder for identity thieves. Eliminating the geographical component of the first three digits makes it more difficult for crooks to reconstruct potential victims’ numbers using publicly available data like an address.
  2. It extends the longevity of the nine-digit Social Security number. The old system limited the range of numbers that could be issued in a given state. When randomization was proposed in 2007, some states had fewer than 10 years’ worth of unassigned numbers.

Now, anyone from anywhere in the country can get one of the more than 400 million available nine-digit combos, giving the format a considerably longer life.

The new system also expanded the pool of numbers by allowing three-digit codes that were not used in the past as area numbers to start off SSNs, including some in the 700s and 800s. The numbers 000, 666 and 900–999 remain off-limits.

No existing Social Security numbers were replaced as a result of the change to randomization. The new system affects only numbers issued since it was put in place.

Keep in mind

  • Randomization will apply if you get a new Social Security number to replace a number assigned before the switch. However, Social Security grants requests to change a number only under very limited circumstances, including if the applicant is a victim of domestic violence or abuse or is suffering significant financial harm from identity theft.

Published February 20, 2020

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