Iowa Code § 210.5: Liquids.

The unit or standard measure of capacity for liquids from which all other measures of liquids shall be derived and ascertained shall be the standard gallon secured in accordance with the provisions of section 210.1. The gallon shall be divided by continual division by the number two so as to make half-gallons, quarts, pints, half-pints, and gills. The barrel shall consist of thirty-one and one-half gallons, and two barrels shall constitute a hogshead.