The metric system, more formally called the meter-kilogram-second (MKS or mks) system of units, is based on three fundamental units: the meter (m) that quantifies displacement , the kilogram (kg) that quantifies mass , and the second (s or sec) that quantifies time . The metric system is formally replaced by the very similar Standard International System of Units ( SI ).
The metric system was originally developed by scientists who were frustrated with the English (foot-pound-second) system. In theoretical and laboratory calculations, arithmetic involving English units is "messy." There are, for example, 12 inches in a foot, three feet in a yard, and 5280 feet in a statute mile; there are 16 ounces in a pound. Less common English units such as the rod, furlong, peck, and bushel seem, in modern scientific terms, to have been dreamed up ages ago without concern for common sense, although they are used by some agricultural and industrial people to this day.
In the metric or mks system, the meter and kilogram are divided into fractional units, and enlarged into multiple units, according to power-of-10 prefix multiplier s. For example, there are 1000 millimeters (10 3 mm) in a meter, 1000 meters (10 3 m) in a kilometer, 1000 grams (10 3 g) in a kilogram, and 1000 milligrams (10 3 mg) in a gram. This makes these mks units easy to work with in scientific notation. Time, however, is denoted in the same way as in the English system. There are 60 seconds in a minute, 60 minutes in an hour, and 24 hours in a mean solar day in both systems.
The Standard International System of Units ( SI ) provides formal definitions for the meter, the kilogram, and the second, and also specifies and defines four additional units: the Kelvin for temperature, the ampere for electric current, the candela for luminous intensity, and the mole for material quantity.