volumes of dry material, each with their own names and sub-units. Although the units and their names are similar to the units in the imperial system, and many units are shared between the two systems as a whole; with respect to volume, however, this is quite the contrary. The independence of the U.S. from the British Empire decades prior to the reformation of units in 1824 — most notably the gallon, its subdivisions, and (in mass) higher combinations above the pound — is the cause of the differences in values. As a non-participant in that reform, the U.S. retained the separate systems for measuring the volumes of liquids and dry material, whereas the imperial system had unified the units for both under a new imperial gallon. Fluid volume One US fluid ounce is of a US pint, of a US quart, and of a US gallon. The teaspoon, tablespoon, and cup are defined in terms of a fluid ounce as , , and 8 fluid ounces. The fluid ounce derives its name originally from being the volume of one ounce avoirdupois of water, but in the US it is defined as of a US gallon. Consequently, a fluid ounce of water weighs about 1.041 ounces avoirdupois. The saying, "a pint's a pound the world around," refers to 16 US fluid ounces of water weighing approximately (about 4% more than) one pound avoirdupois. An imperial pint of water weighs a pound and a quarter (20 oz). There are varying standards for barrel for some specific commodities, including 31 gallons for beer, 40 gallons for whiskey or kerosene, and 42 gallons for petroleum. The general standard for liquids is 31.5 gal or half a hogshead. The common 55-gallon size of drum for storing and transporting various products and wastes is sometimes confused with a barrel, though it is not a standard measure. In the U.S., single servings of beverages are usually measured in fluid ounces. Milk is usually sold in half-pints (8 fluid ounces), pints, quarts, half gallons, and gallons. Water volume for sinks, bathtubs, ponds, swimming pools, etc., is usually stated in gallons or cubic feet. Quantities of gases are usually given in cubic feet (at one atmosphere). Minims, drams, gill, and pottle are rarely used currently. The gill is often referred to as a "half-cup". The pottle is often referred to as a "half-gallon". Dry volume Dry volume is measured