There are 1,000,000,000 possible nine‑digit Social Security numbers in pure theory, because each of the 9 positions can be any digit 0–9, giving 109=1,000,000,00010^9=1{,}000{,}000{,}000109=1,000,000,000 combinations.

However, not all of these are actually usable in the real U.S. system:

  • Valid SSNs have the pattern XXX‑XX‑XXXX, but numbers with any block of all zeros (000‑xx‑xxxx, xxx‑00‑xxxx, xxx‑xx‑0000) are not used.
  • The first three digits (the “area number”) cannot be 000, 666, or in the range 900–999 under current rules.

Because of these restrictions, the number of valid SSNs is somewhat less than 1 billion, but when people ask “how many possible Social Security numbers are there,” they usually mean the mathematical maximum of 1 billion different nine‑digit combinations.