Kilogram

The kilogram (also spelled kilogramme) is the base unit of mass in the International System of Units (SI), equal to one thousand grams. It has the unit symbol kg. The word "kilogram" comes from the metric prefix kilo- (meaning one thousand) combined with "gram," and it is often shortened to "kilo" (plural "kilos").

Originally defined in 1795 during the French Revolution as the mass of one litre of water, the kilogram has evolved over time. In 1889, a special cylinder made of platinum–iridium, called the International Prototype of the Kilogram (IPK), became the standard for over 130 years. Today, the kilogram is defined using three important scientific constants: a specific transition frequency of the caesium-133 atom, the speed of light, and the Planck constant. This modern definition ensures that the kilogram can be precisely measured anywhere in the world using advanced tools like a Kibble balance in well-equipped metrology laboratories.

Definition

The kilogram is the base unit of mass in the International System of Units (SI). It is defined using three important constants: a specific atomic transition frequency that helps define the second, the speed of light in a vacuum which helps define the metre, and the Planck constant which, together with the metre and second, defines mass.

Since 2019, the kilogram has been defined by fixing the value of the Planck constant. This new definition connects the kilogram to fundamental constants of nature, making the unit more precise and unchanging over time. This definition keeps the kilogram very close to its old meaning — about the mass of one litre of water at a specific temperature.

Name and terminology

The kilogram is the only base SI unit with an SI prefix (kilo) in its name. The word kilogram comes from the French kilogramme, which combines the Greek word for "a thousand" with a term for "a small weight". This French term was officially used in 1795.

Both spellings, kilogram and kilogramme, are used, with kilogram being more common in the United States. In the 19th century, the short form kilo became popular in English, though some official guidelines do not recommend it for scientific writing.

Redefinition based on fundamental constants

Main article: 2019 revision of the SI

The kilogram was once defined by a special metal object called the International Prototype of the Kilogram. However, scientists noticed that this object's mass was slowly changing over time. To solve this problem, they decided to define the kilogram using a fundamental constant of nature called the Planck constant. This new definition began on May 20, 2019.

This change means that the kilogram is now linked to unchanging parts of the universe, making measurements more accurate and consistent everywhere in the world. Before this, the kilogram was tied to physical objects, which could change or get damaged over time.

SI multiples

Main article: Orders of magnitude (mass)

"Milligram" redirects here. For the American band, see Milligram (band). For the Serbian band, see Miligram (band). For the horse, see Milligram (horse).

In the International System of Units (SI), we add prefixes to the unit "gram" instead of "kilogram" because the kilogram already has a prefix in its name. For example, one-millionth of a kilogram is called 1 mg, which stands for milligram, rather than using a term like microkilogram. This helps keep things simple and clear when talking about very small or very large masses.

Usage and practical issues with SI mass units

In healthcare, it’s important to be clear about measurements to avoid mistakes. For example, some medicine errors happen when people mix up milligrams and micrograms. In the United States, doctors and nurses must use “mcg” instead of the symbol “μg” to prevent confusion.

Different countries sometimes use special names for weight. In Italy, people often use the hectogram, which is 100 grams, especially when selling food like fruits and vegetables. In some parts of Europe, you might still see “dkg” for 10 grams, used for foods like cheese and meat. For very heavy things, most people use the word “tonne” instead of “megagram,” even though both mean the same thing. The tonne was officially accepted for use with the SI system in 1879.