What Is the International System of Units (SI)?
Definition
International System of Units (SI): The shared system of measurement built on seven base units: the meter, kilogram, second, ampere, kelvin, mole and candela. The International Bureau of Weights and Measures (BIPM) publishes it.
The International System of Units (SI) is the shared system of measurement used in science, trade and everyday life. It has seven base units: the meter (m), kilogram (kg), second (s), ampere (A), kelvin (K), mole (mol) and candela (cd). Since 20 May 2019, all SI units are defined in terms of constants of nature with fixed values.
Where it appears in the formulas
The unit converters use SI units as the common reference. Each factor of the length converter is the number of meters in one unit; each factor of the weight converter is the number of grams. For example 1 in = 0.0254 m and 1 lb = 0.45359237 kg, both exact by definition. The temperature converter uses the definition that links the kelvin and the degree Celsius.
Units outside the SI
The inch, foot, mile, pound and ounce are not SI units. Since the 1959 international yard and pound agreement they are defined exactly in SI units, so their conversion factors are exact.