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Early Babylonian and Egyptian records, as well as the Bible, indicate that weight was originally measured by the capacities of containers such as gourds or clay or metal vessels. These were filled with plant seeds that were then counted to measure the volumes. With the development of scales as a means of weighing, seeds and stones served as standards.
The Babylonians invented the talent as the basic unit of weight and, based on their sexagesimal (60-based) system, divided it into equal parts. It was equal to the amount of water that filled an amphora (a kind of vase).
The Greeks used the same weight measurement as the Babylonians, but the Romans changed it. Their basic unit of weight was the 'uncia,' from which the English word 'ounce' is derived. The uncia is a twelfth part of the 'pes,' which is the Roman 'foot' — our word inch is also derived from 'uncia.'
For the Arabs, a silver Dirhem was determined by 45 full-grown barley grains. Ten Dirhems made a Wukryeh of 450 grains, which we call an ounce from the Latin "uncia" or twelfth.
Metric weights have a different history. The gram and kilogram were developed by scientists in 18th century France. In 1791, the French parliament imposed the use of the metric system on the country, and it is now used in Continental Europe and many other parts of the world.
The metric system was not particularly popular in revolutionary France, and Emperor Napoleon abolished it. But, when Napoleon lost power, the system was reinstated.
The kilogram is the weight of one liter of water. One thousandth of a kilogram is a gram. All multiples and submultiples of the base units are in powers of ten, making conversion between units particularly easy.
To convert kilograms to pounds, multiply the kilogram value by 2.20462. For example, 1 kg = 2.20462 lbs.
There are 14 pounds in a stone. To convert stones to pounds, multiply the stone value by 14.
Mass is a measure of the amount of matter in an object, while weight is the force exerted on that mass by gravity. On Earth, we often use these terms interchangeably, but they are different physical quantities.
The US continues to use the imperial system inherited from British colonial times, while the UK has officially adopted the metric system for most purposes, though imperial units are still used in certain contexts.