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- Different Unit Systems
- The Pound's History
- An Overview of the Metric System's History
- The International System of Units (SI)

Using this Unit Converter, you can easily convert between various units of measurement. Select the current unit of measure in the left column, the desired unit of measure in the right column, and enter a value in the left column to perform the conversion.

The term "system of units" refers to a set of rules governing the relationship between various units of measurement. Humanity has used many systems of units throughout history. A unit of measure is a specific quantity value that is used as a standard for measuring the same type of quantity, such as weight, length, and volume.

It is pretty challenging to communicate in trade or science if you and your business or scientific partners use different systems of units. In the past, many measurement systems were locally determined. They could be based on arbitrary factors, such as the length of the king's thumb. As a result, humanity gradually created more universally applicable and reliable systems.

Today we use the metric, imperial, and conventional unit measurement systems.

SI (The International System of Units) is the most widely used metric system, including seven basic units for length, mass, time, temperature, electric current, luminous intensity, and substance quantity.

Although SI is universally applied in science (even in the United States), some countries, such as the United States, continue to use their own unit systems. This is partly because of the high financial and cultural costs of altering a measuring system versus the potential benefits of employing a standardized approach.

Several unit converters, such as this Conversion Calculator, exist and will continue to exist to ensure that people worldwide can adequately convert different measures.

Arab civilization developed in the Middle East and Spain throughout the eighth and ninth centuries AD. The weight of a minted coin could not simply be chopped or shaved to lower its weight, so the Arabs used coins as a measure. As a basic measure of weight, they used a coin, a silver dirham, the weight of which was approximately equal to 45 fully grown grains of barley.

Over time, trade moved from the Mediterranean to Europe, particularly the northern German City-States. As a result, a pound of silver, 16 ounces, or 7,200 grains, became a widely used unit of measurement in many areas. England also adopted this measure.

Later, Offa, king of Mercia, the kingdom of Anglo-Saxon England, who ruled it from 757 to 796, carried out a monetary reform. He reduced the pound’s size to 5,400 grains to use smaller coins because of silver scarcity. When William the Conqueror ascended to the throne of England, he kept the 5,400-grain pound for coinage. Still, he used the 7,200-grain pound for all other purposes.

Many nations used the pound from that point on, including England. However, during the reign of Queen Elizabeth in the 16th century, the avoirdupois weight system was established. It was a coal-weight-based system with a name derived from the French word "avoir de pois" (goods of weight or property). Avoirdupois was equal to 7,000 grains, 256 drams of 27,344 grains, or 16 ounces of 437 ½ grains. In most English-speaking nations, the avoirdupois pound has been officially defined as 0.45359237 kilograms since 1959.

Asian countries have also seen the development of different measurement techniques. To illustrate this point, in ancient India, a unit of weight known as the "Satamana," or 100 gunja berries, was used.

Shi Huang Di, the first Chinese emperor, instituted a system of weights and measures around the third century BCE (Before the Common Era). The shi, or 132 pounds, was used as the standard unit of weight measurement. According to Chinese tradition, chi and zhang were the units of length, equal to roughly 25 centimeters and 3 meters.

Another method developed in China to ensure accuracy was to use a bowl of a specific size that made a distinct sound when struck. Measurement was not considered correct if the sound produced was off-tune.

In 1668, John Wilkins, a natural philosopher, author, and one of the founders of the Royal Society, proposed a decimal system. In his system, length, area, volume, and mass were related based on a pendulum with a beat of one second as the basic unit of length.

In 1670, Gabriel Mouton, a French abbot and scientist, proposed a decimal system based on the earth’s circumference. This idea was supported by other prominent scientists, such as Jean Picard and Christian Huygens. Still, it did not catch on for another 100 years or so.

The standardization of measures and weights became apparent to countries trading and exchanging scientific ideas by the mid-eighteenth century.

Charles Maurice de Talleyrand-Périgord, Prince Talleyrand, suggested using the pendulum’s length to establish a uniform measurement standard. One of France's most influential scientific bodies of the time offered a decimal weight and measure system similar to the committee founded in the United States.

As a part of his "Plan for Establishing Uniformity in the Coinage, Weights, and Measures of the United States," Thomas Jefferson proposed a decimal system in which each unit was a multiple of 10. Congress considered Jefferson's report, but took no action on his recommendations.

In 1795, French legislation officially defined the metric system. By 1799, the metric system was formally adopted in France, although not all citizens followed it.

The metric system did not expand fast, and France’s regions captured during Napoleon’s rule were the first to adopt it. By 1875, two-thirds of the European and almost half of the world’s population accepted the metric system. By 1920, 22 percent of the world’s population used the imperial or U.S. customary systems, 25 percent primarily used the metric system, and 53 percent used neither.

In 1960, the International System of Units was created, making it the most commonly used measurement system. Except for the United States, all industrialized countries have adopted it. In the U.S., the army and science use it extensively.

The International System of Units for Physical Units was adopted in 1960 by the 11th General Conference on Weights and Measures in Paris.

In 1948, the International Union of Pure and Applied Physics proposed developing a unified International System of Units. As a result, the SI system was created to simplify the use of units of measurement. The system was adopted as the basic system of units by most countries in the world.

In those countries where traditional units are still used in everyday life, their definitions have been changed to link them with SI units.

The SI system is based on the principles first applied in 1832 by the mathematician Carl Gauss in constructing the Gaussian system of units. The essence of Gauss's method is that, initially, definitions of dimensions are established only for a few basic units that are independent of each other. And the other units connected with them are regarded as their derivatives.

The basic units of the SI became:

The meter (unit of length), the kilogram (unit of mass), the second (unit of time), and the ampere (unit of electric current), the Kelvin (unit of temperature), and the candela (unit of light intensity). In 1971, the unit of quantity of matter, the mole, was added to the basic units.

Within the SI, these units are considered to have independent dimensions. None of the basic units could be derived from the others. The three basic units (meter, kilogram, and second) allow the formation of derivative units for all quantities having a mechanical nature.

Some derived units in the SI system were named after the scientists. They are Hertz, Newton, Pascal, Joule, Watt, Coulomb, Volt, Farad, Ohm, Siemens, Weber, Tesla, Henry, Celsius, Becquerel, Gray, Sievert, and Katal.

The SI system adopts a set of special prefixes: deca, hecto, kilo, mega, giga, deci, centi, milli, micro, nano, etc. They are used when the values of the quantities being measured are much larger, or much smaller, than the SI unit used without the prefix. They mean to multiply or divide a unit by a certain integer, the power of 10. For example, the prefix "kilo" means to multiply by 1000 (a kilometer = 1000 meters). SI prefixes are also called decimal prefixes.

The SI system does not cover all popular units of measurement. It does not include the minute, hour, day, angular degree, angular minute, angular second, hectare, liter, ton, electronvolt, bar, millimeter of mercury, angstrom, mile, and others. When using such units, scientists apply coefficients to convert these units to SI.

The system does not stand still and periodically updates the criteria by which quantities are measured due to advances in scientific knowledge. The definition of a second in the SI system was changed in 1967, the definition of a candela in 1979, and the definition of a meter in 1983. Scientists had also been working to redefine the kilogram, ampere, kelvin, and mole as their definitions were based on physical artifacts.

For example, the kilogram used to be defined by a real physical standard, a platinum-iridium cylinder made in 1889 and stored at the International Bureau of Weights and Measures in Paris. However, as scientists discovered, its mass gradually decreased. Therefore, the value of the kilogram began to be defined by Planck's constant, a coefficient that relates the magnitude of the energy of a quantum of electromagnetic radiation to its frequency.

Previously, a meter in the SI system was equal to 1/10,000,000 of the distance from the North Pole to the equator. In the modern SI system, a meter is a distance traveled by light in a vacuum in 2997924583 seconds. Before the last revision, a second was defined as a day divided by 24, 60, and 60. Nowadays, a second is equal to 9192631770 periods of radiation of the cesium atom during the transition between the ground state levels of cesium.