The metric system is a decimal-based system of measurement originally based on the meter and kilogram, which were introduced by France in 1799. "Decimal-based" means all the units are based on powers of 10. There are the base units and then a system of prefixes, which may be used to change the base unit by factors of 10. Base units include the kilogram, meter, and liter (liter is a derived unit). Prefixes include milli-, centi-, deci-, and kilo. The temperature scale used in the metric system is the Kelvin scale or Celsius scale, but prefixes are not applied to degrees of temperature. While the zero point is different between Kelvin and Celsius, the size of the degree is the same.

Sometimes, the metric system is abbreviated as MKS, which indicates the standard units are the meter, kilogram, and second.

The metric system often is used as a synonym for SI or the International System of Units, since it is used in nearly every country. The major exception is the U.S., which approved the system for use back in 1866, yet has not switched over to SI as an official measurement system.

## List of the Metric or SI Base Units

The kilogram, meter, and second are the fundamental base units upon which the metric system is built, but seven units of measure are defined from which all the other units are derived:

- Kilogram: The kilogram (kg) is the base unit of mass.
- Meter or Metre: The meter (m) is the unit of length or distance.
- Second: The second (s) is the fundamental unit of time.
- Kelvin: The Kelvin (K) is the metric unit of temperature.
- Mole: The mole (mol) is a unit for a quantity of a substance.
- Ampere: The Ampere (A) is the unit of electric current.
- Candela: The candela (cd) is the unit of luminous intensity. The candela is sometimes called by its old name, the candle.

The names and symbols for the units are written with lowercase letters, except for Kelvin (K), which is capitalized because it was named in honor of Lord Kelvin, and Ampere (A), which is named for Andre-Marie Ampere.

The liter or litre (L) is an SI derived unit of volume, equal to 1 cubic decimeter (1 dm^{3}) or 1000 cubic centimeters (1000 cm^{3}). The liter actually *was* a base unit in the original French metric system but is now defined in relation to length.

The spelling of liter and meter may be litre and metre, depending on your country of origin. Liter and meter are American spellings; most of the rest of the world uses litre and metre.

## Derived Units

The seven base units form the basis for derived units. Still more units are formed by combining base and derived units. Here are some important examples:

- Radian (rad): Unit used to quantity an angle: m⋅m
^{−1} - Hertz (Hz): Used for frequency: s
^{−1} - Newton (N): Unit of weight or force: kg⋅m⋅s
^{−2} - Joule (J): Unit of energy, heat, or work: kg⋅m
^{2}⋅s^{−2} - Watt (W): Unit of power or radiant flux: kg⋅m
^{2}⋅s^{−3} - Coulomb (C): Unit of electric charge: s⋅A
- Volt (V): Unit of electric potential or voltage: kg⋅m
^{2}⋅s^{−3}⋅A^{−1} - Farad (F): Unit of capacitance: kg
^{−1}⋅m^{−2}⋅s^{4}⋅A^{2} - Tesla (T): Metric unit of magnetic flux density: kg⋅s
^{−2}⋅A^{−1} - Degree Celsius (°C): Temperature relative to 273.15 K.
- Gray (Gy): Unit of absorbed radiation dose: m
^{2}⋅s^{−2}

## The CGS System

While the standards of the metric system are for the meter, kilogram, and liter, many measurements are taken using the CGS system. CGS (or cgs) stands for centimeter-gram-second. It is a metric system based on using the centimeter as the unit of length, gram as the unit of mass, and the second as the unit of time. Volume measurements in the CGS system rely on the milliliter. The CGS system was proposed by German mathematician Carl Gauss in 1832. Although useful in science, the system did not gain widespread use because most everyday objects are more readily measured in kilograms and meters, rather than in grams and centimeters.

## Converting Between Metric Units

In order to convert between units, it's only necessary to multiply or divide by powers of 10. For example, 1 meter is 100 centimeters (multiply by 10^{2} or 100) and 1000 milliliters is 1 liter (divide by 10^{3} or 1000).