The cubic inch and the cubic foot are used as units of volume in the United States, although the common SI units of volume, the liter, milliliter, and cubic meter, are also used, especially in manufacturing and high technology. One cubic inch is approximately 16.387 mL. One U.S. gallon is equal to exactly 231 cubic inches because 3 * 7 * 11 = 231. The cubic inch was established decades ago as the conventional unit in North America for measuring the volume of electrical boxes. Because of the extensive export of electrical equipment to other countries, some usage of the non-SI unit can be found outside North America.