Kilogram (kg)
The kilogram is the base unit of mass in the International System of Units and the unit most people around the world use every day without thinking about it. It is what your bathroom scale reads, what the supermarket labels show, and what doctors use to track your health. Roughly equal to 2.2 pounds, the kilogram bridges the gap between metric and imperial systems in international trade, science, and everyday commerce. For most of its history the kilogram was defined by a single physical object stored in a vault near Paris, making it the last SI unit tied to a human-made artifact. That changed in 2019 when it was redefined using a fundamental constant of nature.
Definition
The kilogram is defined by fixing the numerical value of the Planck constant to exactly 6.62607015 times ten to the power of minus 34 joule seconds. Combined with the existing definitions of the second and the metre, this determines the kilogram without reference to any physical object. In practical terms, one kilogram equals one thousand grams, approximately 2.20462 pounds, or about 35.274 ounces.
History
The kilogram traces its origins to the French Revolution. In 1795 the new republic defined it as the mass of one cubic decimetre of water, roughly the amount that fills a one-litre container. By 1799 a platinum prototype was cast to serve as the physical standard. In 1889 this was replaced by a cylinder made of 90 percent platinum and 10 percent iridium, known as the International Prototype of the Kilogram or Le Grand K. It was stored under three glass bell jars at the International Bureau of Weights and Measures near Paris. Every kilogram measurement in the world was ultimately traceable to this single object. Over the decades, however, scientists discovered that Le Grand K appeared to have lost about 50 micrograms compared to its official copies, roughly the mass of an eyelash. This instability drove a decades-long effort to redefine the kilogram. On 20 May 2019, World Metrology Day, the kilogram was officially redefined using the Planck constant, freeing it from dependence on any physical artifact.
Common Uses
The kilogram is the standard unit for measuring body weight in most countries outside the United States. Supermarkets price produce and meat per kilogram. Gyms label weights in kilograms. Airlines set baggage limits in kilograms, typically 23 kg for a checked bag on international flights. In science, the kilogram is essential for expressing quantities in physics, chemistry, and engineering. Pharmaceutical manufacturing relies on kilogram-scale measurements for batch production.
Did You Know? Facts About Kilogram
- Le Grand K, the original prototype kilogram, is a cylinder of platinum-iridium alloy about the size of a golf ball. It is still kept under three glass bell jars near Paris, though it no longer defines the kilogram.
- Over its lifetime, Le Grand K lost about 50 micrograms compared to its official copies. The cause remains unknown, but it is roughly the mass of a single eyelash.
- The kilogram is the only SI base unit whose name starts with a prefix. Because of this, metric sub-multiples like milligram and microgram are built from "gram" rather than "kilogram" to avoid doubled prefixes.
- Before the 2019 redefinition, every time Le Grand K was cleaned or handled, the mass of every kilogram in the world technically changed along with it.
- The Kibble balance, the instrument used to realize the new kilogram definition, was invented by British physicist Bryan Kibble at the National Physical Laboratory in 1975.