Cubic Inch (cu in)

The volume of a cube one inch on each side, and the dimension in which English liquid measures were legally defined.

Before the imperial system, English gallons were not defined by weight or by any physical standard vessel but geometrically, in cubic inches. The wine gallon was fixed at 231, the ale gallon at 282, and the difference between the two was simply a difference of numbers on a statute. The 1824 Act broke with the practice entirely, defining the imperial gallon instead as the volume of ten pounds of distilled water, which worked out at 277.42 cubic inches. The unit survives in engine displacement figures, where American motoring still speaks of a 350 cubic inch V8.