231 = 3 x 7 x 11. It is the 22nd number that is the product of three distinct primes.
231 is 11100111 in base 2 (binary).
231 is the number of integer partitions of 16.
231 is the 21st triangular number. It is the smallest triangular number in which the sum of the external digits equals the sum of the internal digits.
231 is a hexagonal number and an octahedral number.
231 = 22 + 32 + 72 + 132.
The U.S. liquid gallon is defined as 231 cubic inches.
Source: Number Gossip