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A computer image of the International Prototype Kilogram. It is the kilogram. It sits next to an inch-based ruler for scale. Like the other prototypes, the edges of the IPK have a four-angle to minimize wear (although only three can be seen in this image).
Standard: SI base unit
Quantity: Mass
Symbol: kg
Expressed in: 1 kg =
Natural units 5.609×1029 MeV/c2
Imperial units 2.20462262 lbs

The kilogram or kilogramme[Note 1] is a metric unit of mass. The official kilogram is the mass of one piece of platinum-iridium metal kept in Paris. The piece of metal is known as Prototype Kilogram[1] (IPK).[Note 2] It is now the only metric unit which is defined as an object.[2]

There are attempts to define the kilogram in other ways. One example specifies a number of atoms of a certain substance (at a certain temperature).

One kilogram is a little more than 2.2 pounds. One tonne is one thousand kilograms. One litre of water weighs almost exactly one kilogram, at 3.98° centigrade, at sea level. This was the basis of the definition of the gram in 1795.


In 1879, the piece of metal was made. It was officially chosen to be the kilogram in 1889. It was made of 90% platinum and 10% iridium.[3] Those metals were chosen because they do not rust or corrode like most metals. It is stored in a vault at the BIPM in Sèvres, France. From 1795 to 1799, the unit of mass was not called "kilogram" but was called "grave".

The original kilogram is kept inside bell jars. Over time, dust can collect on it. Before it is measured, it is cleaned to get the original size.[3]

Kilograms and mass

The chains on the swing hold the child’s weight. If one were to stand behind her and try to stop her, one would be acting against her inertia. This inertia comes from her mass, not weight.

The kilogram is a unit of mass. In normal language, measuring mass defines how heavy is something. This is not scientifically correct. Mass is an inertial property. It measures the tendency of an object to stay at a given speed when no force acts on it.

Sir Isaac Newton’s laws of motion contain an important formula: F = ma. F is force. m is mass. a is acceleration. An object with a mass (m) of one kilogram will accelerate (a) at one meter per second per second when acted upon by a force (F) of one newton. This about one-tenth the acceleration due to earth’s gravity.[Note 3]

The weight of matter depends on the strength of gravity. The mass of matter does not. The mass of an object is the same everywhere.[Note 4] Objects are "weightless" for astronauts in microgravity. However, the objects still have their mass and inertia. Astronaut must use ten times as much force to accelerate a ten-kilogram object at the same rate as a one-kilogram object.

A common swing, as shown in the picture, can show the relationship of force, mass and acceleration. Someone could push an adult on the swing. The adult would accelerate slowly. They would only swing a short distance forward before the swing would change direction. If a child is sitting on the swing, then the child would swing forward faster and further.


There are various copies of the original Kilogram. Some of these copies have gained or lost some mass. This is a problem for scientists and engineers who need to make exact measurements.[source?]


  1. The spelling kilogram is used by the International Committee for Weights and Measures (CIPM) and the U.S. National Institute of Standards and Technology (NIST). Sometimes the spelling kilogramme is used in British English.
  2. Also known by its French-language name Le Grand K.
  3. In professional metrology (the science of measurement), the acceleration of earth’s gravity is taken as standard gravity (symbol: gn). The acceleration is really 9.80665 meters per square second (m/s2). The expression "1 m/s2 " means that for every second that elapses, velocity changes an additional 1 meter per second. In more familiar terms: an acceleration of 1 m/s2 can also be expressed as a rate of change in velocity of precisely 3.6 km/h per second (≈2.2 mph per second).
  4. Matter has invariant mass assuming it is not traveling at a relativistic speed with respect to an observer. According to Einstein’s theory of special relativity, the relativistic mass (apparent mass with respect to an observer) of an object or particle with rest mass m0 increases with its speed as M = γm0 (where γ is the Lorentz factor). This effect is vanishingly small at everyday speeds, which are by orders of magnitude less than the speed of light, but becomes noticeable at very high speeds. For example, traveling at just 10% the speed of light with respect to an observer—exceedingly fast compared to everyday speeds (about 108 million km/h or 67 million mph)—increases an object’s relativistic mass just over 0.5%. As regards the kilogram, relativity’s effect upon the constancy of matter’s mass is simply an interesting scientific phenomenon that has zero effect on the definition of the kilogram and its practical realizations.