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What Is a Kilogram?

A kilogram is the base unit of mass in the International System of Units (SI), defined by a physical constant known as the Planck constant. It's equivalent to the mass of the International Prototype of the Kilogram, a platinum-iridium cylinder stored in France. But how does this relate to everyday objects? Discover the surprising connections in the full article.
Cindy Quarters
Cindy Quarters

A kilogram (kg) is a unit of measure that technically is used to specify the mass of an object, but in more common usage it is simply a measure of weight. It is part of the International System of Units (SI), the standard for the metric system. The SI is in use in many countries throughout the world and has many advantages that make it easy to work with.

The metric system is made up of basic units, which includes the gram and the meter. It also has a series of prefixes that can be applied to the base unit to specify quantity. Some of the most common prefixes that refer to units consisting of more than one unit are kilo- and mega-, which mean thousand and million, respectively. A kilogram is a unit of weight that is equivalent to 1,000 grams, while a megagram equals one million grams.

Scientist with beakers
Scientist with beakers

Units smaller than the basic unit to which they refer use another set of prefixes. Some of those that are more commonly used are centi-, meaning one hundredth, and milli-, meaning one millionth. These are used with very small amounts of weight, such as with a centigram, which indicates that something weighs one one-hundredth of a gram, or the centimeter, a length equal to one one-hundredth of a meter.

Since not all countries use the metric system, there are times when it is necessary to convert from the metric system to the avoirdupois system, which is used in the United States. The avoirdupois system can be difficult to work with, since it is based on pounds, each of which weighs 16 ounces; this can be more awkward than the consistent units of 10 in the metric system. The scientific community and many other applications all rely on the metric system, so conversions are common.

Switching between the metric and avoirdupois system requires a simple conversion. There are roughly 2.2 pounds in a kilogram. For example, if a person wanted to know how many pounds are in 100 kilograms, simply multiply the number of kg, 100, by the number of pounds in one kilogram, 2.2, to determine that there are 220 pounds in 100 kg.

Conversely, to determine how many kilograms are in a certain number of pounds, multiply the number of pounds by the number of kilograms in a pound. This value is 0.45. Using this value, there are 4.5 kilograms in 10 pounds.

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      Scientist with beakers