The basic unit of mass in the current metric system and the International System of Units (SI) is the kilogram. Its unit symbol is kg. It is a widely used unit in the field of science, engineering, and commerce all across the globe, and it is often even called a kilo in everyday speech. A kilogram was originally defined in 1795 to be equal to the mass of a thousand cubic cm or one liter of water. One kilogram is said to be 2.2 times heavier than a pound. A pound is defined to be exactly equal to 0.45359237 kg.

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### The International System of Units

During the initial times, the Meter convention was just concerned with the units, kilogram, and meter, but the scope of the treaty extended in 1921 to accommodate all the physical measurements and thus all the aspects present in the metric system. In 1960, the 11th CGPM held in Paris approved the International System of Units.

Just as the initial formulation of the metric system had grown out of the confusions scientists came across in dealing with the medieval system, so a new system grew out of the problems an enormously large scientific community faced in the rapid growth of the subsystems improvised to serve certain disciplines. At the same time, it came to the notice of the people that the initial standards of the 18th century were not accurate to the degree demanded by the 20th-century operations so new definitions were required. In 1960 October, after a long discussion, the 11th General Conference on Weights and Measures in Paris defined a new International System of Units. Its abbreviation is SI. The base units for the following were adopted and defined accordingly:

**Length:**meter (m)**Mass:**kilogram (kg)**Time:**second (s)**Thermodynamic Temperature:**kelvin (K)**Electric Current:**ampere (A)**Light (luminous) Intensity:**candela (cd)**Amount of Substance:**mole (mol)

### What is Kg?

As originally defined, the unit of mass kilogram was represented by a solid cylinder of platinum in the late 18th century. The measurement of the mass of the volume of water turned out to be pretty imprecise and inconvenient to be made, making the platinum artifact itself the standard. Later in 1889, it was replaced by a standard kilogram, also called a cylinder, of height equivalent to its diameter, made up of the exact same platinum-iridium alloy which was then used as the standard for determining the unit meter. The standard kilogram was kept in France, at the International Bureau of Weights and Measures at Sevres. However, in 1989 it finally came to notice that the sample kept at Sevres was 50 micrograms lighter than the other copies of the standard kilogram. To avoid the confusion of having the kilogram defined by an object of inconsistent mass, a proposal was agreed to begin to redefine the kilogram not by a physical artifact but by a fundamental physical constant in the year 2011 by the General Conference on Weights and Measures (CGPM). Planck’s constant was the constant that was chosen, which was to be defined as equal to 6.62607015 x 10-34 joule second. One kilogram times meter squared per second squared is said to be equal to one joule.

### Some SI Multiples of Mass

As SI prefixes are not concatenated within the name or symbol for a unit of measurement, SI prefixes are used with the unit of mass, gram, and not kilogram. This is because kilogram already has a prefix as a part of its name. For example, one-millionth of a kilogram is not 1 microgram, but 1 milligram.

**Some Common Multiples of Gram are:**

10-3 gram = 1 milligram

10-6 gram = 1 microgram

10-9 gram = 1 nanogram

10-12 gram = 1 picogram

10 gram = 1 decagram

102 gram = 1 hectogram

103 gram = 1 kilogram

106 gram = 1 megagram (1 tone)

109 gram = 1 picogram

Some Common Facts on Certain Multiples of the Unit Gram are:

The microgram is abbreviated as “mcg” and this unit is used in pharmaceutical and nutritional supplement labelling.

Another very commonly used multiple of the unit gram is hectogram. It is used in the retail food trade in Italy. It is usually called etto which is a short form for ettogrammo which is the Italian for hectogram. 1 hectogram is equivalent to 100 g.

The unit megagram is rarely used. For most purposes, the unit tonne is used. Its symbol is “t”. The unit megatonne is used in general-interest literature on greenhouse gas emissions. The equivalent unit found in science papers on the subject is often teragram (Tg).

In the UK due to some serious medication errors which have occurred due to the confusion between milligrams and micrograms, microgram must be written in full and it is unacceptable to use “mcg” or “µg”.

Q1. Define the SI Unit of Mass.

Kilogram (kg) is the base unit of mass in the International System of Units and the current metric system. As the units like meter and second were already defined in terms of the speed of light and the frequency of a spectral line of cesium respectively, the kilogram hence came to be determined by accurate measurements of Planck’s constant. The proposal came to be accepted at the 2018 General Conference on Weights and Measures (CGPM) and later came to effect from the 20th May of 2019 that the kilogram would be defined by Planck’s constant. Initially the mass of 1 litre of water was said to be exactly equal to 1 kg.

Q2. Explain 1 kg Definition.

The SI unit of the quantity mass is kilogram (kg). It is defined as the mass of a certain sample made up of platinum-iridium and kept at the International Bureau of Weights and Measures in Sèvres, France. At the surface of the Earth, a mass of 1 kg weighs almost 2.20 pounds (lb). Contrariwise, an object whose weight is 1 lb at the surface of the Earth has a mass of approximately 0.454 kg. There is a qualitative and quantitative difference between kilograms and pounds. The unit kilogram denotes mass but weight is denoted by the unit pound.