231

This number is a composite.

231 = 3∙7∙11 = 6 ∙ (7/2)^2 ∙ 22/7, hence 231 in^3 is the volume of a cylinder with height 6 inches and diameter 7 inches. So a gallone wine was defined in 1706 as a block with sides 3 in., 7 in., 11 in. Later this became the 1 US liquid gallon, and since the inch was defined as 2.54 cm (exactly!), we have 1 US liq gal = 231 ∙ 2.543 cm^3, or approximately 3.785 Liter. [Fegert]