Imperial and U.S. Customary Units
In U.S. customary units, most units of volume exist both in a dry and a liquid version, with the same name, but different values: the dry hogshead, dry barrel, dry gallon, dry quart, dry pint, etc. The bushel and the peck are only used for dry goods. Imperial units of volume are the same for both dry and liquid goods. They have a different value from both the dry and liquid US versions.
Many of the units are associated with particular goods, so for instance the dry hogshead has been used for sugar and for tobacco and the peck for apples. There are also special measures for special goods, such as the cord of wood, the sack, the bale of cotton, the box of fruit, etc.
Because it is difficult to measure actual volume and easy to measure mass, many of these units are now also defined as units of mass, specific to each commodity, so a bushel of apples is a different weight from a bushel of wheat (weighed at a specific moisture level). Indeed, the bushel, the best-known unit of dry measure because it is the quoted unit in commodity markets, is in fact a unit of mass in those contexts.
Conversely, the ton used in specifying tonnage and in freight calculations is often a volume measurement rather than a mass measurement.
In U.S. cooking, dry and liquid measures are the same: the cup, the tablespoon, the teaspoon.
US dry measures are 16% larger than liquid measures; this is advantageous when cooking with fresh produce, as a dry pint of vegetables after trimming ends up being about a cooking (liquid) pint.
Read more about this topic: Dry Measure
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