Sixty-six (66, LXVI) is a natural number following sixty-five and preceding sixty-seven. It is a composite number with the divisors being 2, 3, 11, 22 and 33. It is a sphenic number. Its factorization is:
- The atomic number of dysprosium, a lanthanide.
- The designation of the historic Route 66, dubbed the "Mother Road" by novelist John Steinbeck.
- The designation of US Interstate 66, a freeway that runs from Virginia to Washington, D.C.