The International System of Units (SI) is the only system of units that is universally recognised, so it has a distinct advantage for establishing international dialogue. Other units, i.e. non-SI units, are generally defined in terms of SI units using conversion factors.
The use of the SI, as a standard system of units, simplifies the teaching of science. For these reasons, the use of SI units is recommended in all fields of science and technology. However, when non-SI units are used, the correspondence to the SI should always be quoted.
Nonetheless, some non-SI units, such as the minute, hour and day as units of time, continue to be widely used.
Metric System 