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     A pound of air has the same mass and weight as a pound of water, feathers, or almost anything else; so why does an ounce of air (or water or feathers) weigh LESS than an ounce of gold, but a pound of air (or water or feathers) weighs MORE than a pound of gold?
     This sounds like a trick question. After all, if a pound's a pound no matter what you're measuring, then a pound of anything should weigh the same as a pound of anything else, and an ounce of anything should weigh the same as an ounce of anything else. But... there are pounds and ounces and ounces and pounds that are not the same at all; and the pounds and ounces used to measure precious metals such as gold are not the same as those used to measure most other things.
     Precious metals are measured by Troy weight, which goes back to the end of the first millennium and is based on 480 "grains" of weight per ounce, and 12 ounces per 5760 grain pound. The weight of air, water or feathers, however, is based on the Avoirdupois Pound, which was established as 7000 grains in the mid-14th century, and is therefore 20% heavier than a Troy pound; but since there are 16 ounces in an Avoirdupois pound, each ounce is only 437.5 grains, or 10% less than a Troy ounce. So an ounce of air, water, or feathers weighs about 10% less than an ounce of gold, but a pound of air, water, or feathers weighs about 20% more than a pound of gold, because the units of measurement, though they share the same names, are not the same at all.