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Ounces -- they're tricky, and quite often the cause of recipe-reading confusion. Luckily, we already knew that a fluid ounce is a measure of volume, as opposed to a dry ounce (or avoirdupois ounce), which is a measure of weight. Beyond that, we weren't sure of the distinction or how the two different measures came to share the same name. So... We searched Yahoo! for the term "measurements" and were quickly rewarded with an entire category on Measurements and Units. From there, we decided to visit How Many, a "dictionary of units of measurement with
commentaries and explanations." Using How Many's alphabetical index, we easily located multiple entries for the word ounce. First, we'll share the definition for avoirdupois ounce, jam-packed with interesting facts: ...The avoirdupois ounce, the unit commonly used in the United States, is 1/16 pound or about 28.3495 grams.... The word ounce is from the Latin uncia, meaning a 1/12 part, because the Roman pound was divided into 12 ounces. The word "inch," meaning 1/12 foot, has the same root. The symbol "oz" is from the old Italian word onza (now spelled oncia) for an ounce. Next up, the explanation of fluid ounce: A
traditional unit of liquid volume, called the fluid ounce to avoid confusion with the weight ounce. In the U. S. customary system there are 16 fluid ounces in a pint, so each fluid ounce represents 1.804 687 cubic inches or 29.573 531 milliliters. In the British imperial system there are 20 fluid ounces in an imperial pint, so each fluid ounce represents about 1.733 871 cubic inches or 28.413 063 milliliters. A fluid ounce of water weighs just a bit more than one ounce avoirdupois. So, according to these definitions, not only are the two ounces used to measure different properties of a substance, there are two different standards for measuring a fluid ounce. It's enough to make a person beg for the metric system!
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