There is a lot of measurement involved in our daily lives. We measure height, weight, blood pressure, time, air temperature, car speeds, and countless other things. Measurements are especially important in technology. When building a bridge, the strength calculations used must be correct. Similarly, achieving good sound insulation requires familiarity with acoustic properties.

Before the emergence of standardised measurement systems, humans used hundreds of different measurement units. Generally, the starting point of a measurement system was something familiar from everyday life, such as the elbow, foot, shoulder, lap, palm or inch (the distance from the tip of the thumb to its first joint). Of course, such units of measurement were not standardized and accurate. As science began to evolve and international connections increased, there was a desire to be able to assemble machines from parts made around the world. The parts would not have come together in any way if everyone had measured their lengths with their own measurement systems. Therefore, it became necessary to introduce well-defined basic quantities that everyone would use in their measurements. In addition, there was a need to conveniently present very large or very small measurement results.

The starting point of the SI measurement system is that each quantity has one unit, which is used consistently. Some of the quantities are basic units, but most are derived from them.

Basic quantities and units

Quantity	Unit	Symbol
length (l)	metre	m
mass (m)	kilogram	kg
time (t)	second	s
electric current (I)	ampere	A
temperature (T)	kelvin	K
luminous intensity (I)	candela	cd
amount of substance (n)	molw	mol

Derived quantities can be obtained from these basic quantities as a result of division and multiplication. Some derived units have their own name and ID. The separately named derivative units most commonly used in everyday life can be found tabulated in the "Tables and formulas" section.

The international system for checking quantities and units began in 1875. At that time, 17 states signed the International Metric Treaty in Paris. The International Organization for Standardization (ISO) decides on the application of the SI system in various fields. In addition to developing the unit system, standardization means general agreements on the dimensioning and quality of products. For example, tubes and fittings, lamps and lampholders, plugs and sockets, and the size of the sheet of paper and the width of the roll of the typewriter must be compatible. Thanks to the standardization organization, you can be sure that a bearing made on the other side of the world will fit a machine that was made ten years earlier somewhere else.