Kilogram vs. Kilo

Kilogram

The kilogram or kilogramme (symbol: kg) is the base unit of mass in the International System of Units (SI), and is defined as being equal to the mass of the International Prototype of the Kilogram (IPK, also known as “Le Grand K” or “Big K”), a cylinder of platinum-iridium alloy stored by the International Bureau of Weights and Measures at Saint-Cloud, France.

The kilogram was originally defined as the mass of a litre (cubic decimetre) of water at its freezing point. That was an inconvenient quantity to precisely replicate, so in the late 18th century a platinum artefact was fashioned as a standard for the kilogram. That artefact, or an exact replica thereof, has been the standard of the unit of mass for the metric system ever since.

Though the IPK, the current primary artefact, and its replicas are stored in carefully controlled laboratory conditions, their masses have been subject to fluctuation as a result of poorly understood factors, possibly including handling, cleaning and contamination. The IPK has diverged from its replicas by 50 μg since their manufacture late in the 19th century. This has led to calls to replace the artefact with a standard defined in terms of invariant constants of nature.

The avoirdupois (or international) pound, used in both the imperial and US customary systems, is defined as exactly 0.45359237 kg,

making one kilogram approximately equal to 2.2046 avoirdupois pounds. Other traditional units of weight and mass around the world are now also defined in terms of the kilogram, making the IPK the primary standard for virtually all units of mass on Earth.