Conversion Calculator
Use this Conversion Calculator to convert between commonly used units. Select the current unit in the left column, the desired unit in the right column, and enter a value in the left column to generate the resulting conversion — every unit in the category is converted at the same time.
Different Systems of Units
Historically, many different systems of units have been used. A system of units is a collection of units of measurement with rules that relate them to each other, and a unit of measurement is a defined magnitude of a quantity used as a standard for measuring the same kind of quantity — length, weight, volume, and so on. In the past, many systems were defined locally and could rest on factors as arbitrary as the length of a king's thumb. That works locally, but for trade and science a standard others cannot relate to makes interaction difficult, so more universal and consistent systems developed over time. Today the systems in use include the metric system, the imperial system, and United States customary units.
The International System of Units (SI) is the standard metric system, built on seven base units: length, mass, time, temperature, electric current, luminous intensity, and amount of substance. SI is used almost universally in science — including in the US — but some countries, notably the United States, still use their own customary units in everyday life, partly because of the substantial financial and cultural cost of changing compared with the benefit of a standardized system. Because US customary units are so entrenched and SI is already used wherever standardization matters most, everyday use of customary units is unlikely to change — which is why unit converters like this one exist, and will continue to exist, so people everywhere can communicate measurements effectively.
History of the Pound
In the eighth and ninth centuries CE, Arab civilization flourished in the Middle East and Spain. The Arabs used coins as a measure of weight, since a minted coin could not easily be cut or shaved to reduce its weight and thus provided a measurable standard. Their basic measure was a silver coin, the dirhem, weighing roughly 45 fully grown grains of barley. Ten dirhems made a Wukryeh, translated into Latin as "uncia" — the origin of the word "ounce." As trade spread from the Mediterranean to Europe, a pound of 16 ounces of silver, or 7,200 grains, became a common measure in many regions.
England adopted the measure too, but a shortage of silver led King Offa to reduce the pound to 5,400 grains so smaller coins could be used. William the Conqueror later kept the 5,400-grain pound for minting coins but reverted to the 7,200-grain pound for other purposes. The avoirdupois system was adopted during the reign of Queen Elizabeth in the 16th century; based on the weight of coal, its name comes from the French "avoir de pois" (goods of weight). It was equivalent to 7,000 grains, 256 drams, or 16 ounces. Since 1959, the avoirdupois pound has been officially defined in most English-speaking countries as exactly 0.45359237 kilograms.
Other systems developed in Asia as well: in ancient India the "Satamana" equalled the weight of 100 gunja berries, and in third-century BCE China the first emperor Shi Huang Di created a system based on the shi (about 132 pounds), with the chi and zhang as units of length of roughly 25 centimeters and 3 meters. The Chinese even ensured accuracy with a special bowl that made a specific sound when struck — if the sound was off pitch, the measurement was not accurate.
Brief History of the Metric System
In 1668, John Wilkins proposed a decimal system linking length, area, volume, and mass to a pendulum with a beat of one second. In 1670, Gabriel Mouton proposed a decimal system based instead on the circumference of the earth — an idea supported by scientists such as Jean Picard and Christiaan Huygens, though it did not take hold for another hundred years. By the mid-eighteenth century, nations that traded and exchanged scientific ideas clearly needed standardized weights and measures. In 1790, Talleyrand approached the British and the Americans with proposals for a common standard of length based on a pendulum, and Thomas Jefferson presented a plan advocating a decimal system in the United States; a French committee of prominent scientists reached a similar conclusion. The system was implemented only in France, where the metric system was formally defined in law in 1795 and officially adopted in 1799.
Adoption spread slowly: areas annexed by France under Napoleon were the first to follow. By 1875, two thirds of the European population and nearly half the world's population had adopted the metric system. The International System of Units, published in 1960, is now the most widely used measurement system in the world and has been adopted by all developed countries except the United States — though it is used there in science and heavily in the military.