A fluid ounce is a measurement of volume, with 16 fluid ounces equaling one pint. Fluid ounces are measured in standardized, predefined containers. A fluid ounce of water will take up the same amount of space as a fluid ounce of sugar. A dry ounce is a measurement of weight, with 16 dry ounces equaling one pound. A dry ounce of sugar weighs the same as a dry ounce of water, but will take up a different volume of space.
The terms fluid and dry are misleading because they can mistakenly cause people to assume that a fluid ounce is for measuring liquids, and dry ounces are for measuring solids. Fluid ounces are used to measure dry goods every day, and dry ounces are used to measure liquids.
Consider the familiar kitchen measuring cup delineated in fluid ounces. Home recipes call for a certain volume of sugar, cream, oil, water or flour. The weight of each of these ingredients is irrelevant, and thankfully so. It would be very time consuming to make a recipe by weighing out each ingredient!
Some goods don’t lend themselves to the measurement of a fluid ounce and are instead measured by weight using dry ounces. Meat, for example, is sold by weight. Even so, any two cuts of beef that are both one pound might contain differing amounts of bone, gristle or fat. This gave rise to lean cuts that are accordingly more expensive because there is more meat per pound.
Items like breakfast cereals and chips are sold by weight so that every box or bag has the same amount of food. Contents normally settle in shipping and in opening an item like this it can appear as if the box or bag is short. For this reason manufacturers added a printed statement to many of these products informing consumers the item is sold by weight and not volume — another way of saying it is sold by dry ounces verses the fluid ounce.
When speaking of dry or fluid ounces most people simply say “ounces.” The type of measurement — weight or volume — is usually clear from the context. Buying a pound of sugar, we understand it will be 16 [dry] ounces by weight. When making cookies and a recipe calls for eight ounces of sugar, however, we don’t add half a pound! Instead the sugar goes into a measuring cup for eight [fluid] ounces, or one cup.
One fluid ounce in the U.S. is equal to 1.04 fluid ounces in the U.K. A Troy ounce, used to measure precious metals like gold, platinum and silver, is 480 grains, or 31.10 grams. This is a few grams more than a standard U.S. dry ounce, which is 28.35 grams.