Measurements in Science is the process of obtaining the magnitude of a physical quantity and expressing it relative to a certain basic, arbitrarily chosen, internationally accepted reference standard called UNIT.

We need only a limited number of units to express all the physical quantities because these quantities are dependent on each other. We assign a set for fundamental or base quantities only, the units for these are called the Fundamental/Base units.

All remaining quantities can be derived from the base quantities. The units for these derived quantities are called derived units.

### A system of units = Fundamental Units + Derived Units

There are different systems of units e.g. CGS(centimeter, gram and second), MKS(meter, gram and second) etc. However, the most commonly used standard is the SI system.

# SI System

#### 7 Base quantities

 Base Quantity Name Symbol Length metre m Mass kilogram kg Time second s Electric Current ampere A Thermodynamics Temperature Kelvin K Amount of substance mole mol Luminous Intensity candela cd