Introduction and Overview
Units of common measurement vary widely, from time to time, and place to place. Most units of measurement started as a reference to a physical object or concept. The foot was the length of a man's foot, the inch was the width of a man's thumb, a furlong was the length of a plowed furrow in a field, an acre was the amount of land a man and two oxen could plow in a day, etc. At first, most measurements were only approximations, but eventually many country's governments set each at a specific standard to make commerce possible and fair. Often, when people settled new lands, they used the names of old measurements, but set their own standards. Other times, similar sounding measurement names in different countries had greatly different values. Some measurements were derived from other types of measurements, such as a barrel weight being the weight of a barrel of flour. Often, the same measurement had different values depending on the material being measured, such as a wine tun and a beer tun, or a hank of wool and a hank of cotton. These differences made sense to the people that used them, but they seem odd today.
A Frenchman first defined what he called the 'meter' as one ten millionth of the distance from the North Pole to the equator along the Prime Meridian. It was later defined, in a more precise method, as so many wavelengths of a certain color of light. A liter was originally defined as a cubic decimeter, and a kilogram was defined as a liter of pure water at a specified temperature. Later, the standard was changed such that a kilogram mass became the standard and the liter was derived as the volume of a kilogram of water. This has caused the liter to become slightly more than a cubic decimeter.
The International System (SI) was first proposed in France in the 17th century, but was not adopted by France until 1795. The system defined that there was only one standard in each measurement type and each unit greater or less was a power of ten. This made conversions between units much simpler. During the 19th century, several countries made this system their standard, but notably not Britain or the US. In 1965 Britain began changing to the metric system as a condition of membership in the European Common Market. The US government, recognizing the problems of international trade, officially made the metric system its standard in 1975.
Still today, units of common measurements (non-metric) are used throughout the world. It would be hard to forget the foot, yard, mile, quart, gallon, or acre because so many physical objects were based on them. And for convenience sake, it will always be easier to say "a cup" than "two deciliters." It is easier to envision a mile than a kilometer because fence rows, city blocks, and farmland measurements were originally based on the mile.
* Much of our written history still refers to things in common units. The Bible does not refer to meters or kilograms, but to cubits and stadia, or shekels and drachma. Wouldn't it be nice to know what they were talking about way back then? Now you can use megaConverter! This megaConverter is specifically for volume measurements commonly used today in the US, Britain, and by the SI system. See megaConverters Ancient Volume #36 and Foreign Volume #37 for volume conversions common in ancient times and foreign countries. For a more complete listing of ancient, foreign, and obsolete measures, download our 'megaSpreadsheet' of conversions in MS Excel format.
For the most comprehensive treatment of measurements, find "NTC's Encyclopedia of International Weights & Measures" by William D. Johnstone at your local library.
Note: Because of round-off errors, converting from very large units to very small units or vice-versa may not be accurate (or practical). Conversion factors can be found by converting a quantity of 1 unit to another unit several steps above or below the first. You may need to string several conversion factors together to find the factor from a very large unit to a very small unit, and then you can use a calculator with sufficient digits to find your answer.