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What is a Kilogram?

A kilogram is a unit of mass in the International System of Units (SI), having the unit symbol kg. It is a widely used measure in science, engineering and commerce worldwide, and is often simply called a kilo colloquially. It means ‘one thousand grams’.

The kilogram is defined by taking the fixed numerical value of the Planck constant h to be 6.626 070 15 × 10−34 when expressed in the unit J⋅s, which is equal to kg⋅m 2 ⋅s −1, where the metre and the second are defined in terms of the speed of light c and a specific atomic transition frequency Δ νCs.

One kilogram is equal to 2.20462262185 pounds (lbs). To convert kilograms to pounds, divide the mass in kilograms by 0.45359237.

The word kilogram comes from the French kilogramme, which in turn comes from the Greek χίλιοι (chilioi), meaning ‘thousand’, and γράμμα (gramma), meaning ‘small weight’.

History of the Kilogram

The kilogram has a long and fascinating history that spans over two centuries and several continents. The kilogram was born from an effort to create a universal and precise system of measurement that would facilitate trade and commerce.

In 1791, the French National Assembly realized that something had to be done about the chaotic state of French weights and measures. They commissioned a group of scientists to devise a new system based on natural and invariable units. The scientists proposed a decimal metric system, in which the unit of length would be the meter, defined as one ten-millionth of the distance from the equator to the pole, and the unit of mass would be the gram, defined as the mass of one cubic centimeter of water at 4 degrees Celsius.

However, a gram was too small for practical purposes, so they decided to use a thousand times larger unit, the kilogram, as the base unit of mass. The kilogram was originally defined as the mass of one liter of water at the ice point, which was determined to be 18,841 grains.

In 1799, a platinum cylinder called the Kilogramme des Archives was manufactured as a prototype for the kilogram. It replaced the previous definition based on water and became the official standard of mass in France.

In 1875, 17 countries signed the Treaty of the Meter (also known as the Meter Convention) to establish new international prototypes for mass and length. The agreement defined the official unit of mass as the kilogram and embodied it in a new metal artifact made of platinum-iridium alloy, called the International Prototype of the Kilogram (IPK). The IPK was essentially identical to the Kilogramme des Archives and was stored in a vault near Paris. Copies of the IPK were sent to the signatory countries to serve as national standards.

In 1890, the United States received two copies of the IPK, K4 and K20. They served as official national kilogram standards and check standards, along with additional kilogram standards that the U.S. later acquired. They are kept at NIST’s Gaithersburg campus in Maryland.