We carry out measurements in our everyday lives. We can measure our heights, weights, the length and width of a tabletop, the speed of a car, the weight of a book, our body temperature, and so on.
Measuring weight on a scale
The figure above shows a man standing on a scale to measure his weight.
We are able to define a quantity only if there exists an amount with which we can measure our quantity. Such an amount has the role of a measurement standard.
This standard of measurement represents the unit of the quantity.
A unit of measurement is therefore defined as a standard magnitude of a physical quantity that is normally used to express the quantity.
A unit is any measurement of which there is just only 1.
The required amount will then be obtained by taking a multiple of the standard of measurement (unit) of the quantity.
In the world of science, every field involves measurements, which have to be communicated to others in a common language that everyone would understand.
The problem above clearly shows that we need to communicate in a unified system of units. Which was the reason for introducing common units that would be used all over the planet. The set of these units is called the SI system or metric system.
The international system of units (SI units) is the metric system of units generally accepted and used in science all around the world. It is one of the possible units of a quantity.
Since there are two types of physical quantities, there are also two types of units, which are:
base units and
derived units
These are the units of base or fundamental quantities. There are seven base units in the SI system and these are listed in the table below:
These are the units of derived quantities. They are obtained from a mathematical combination of the SI base units.
There are numerous derived units. Some of the units with their quantities and derivations are listed in the table below:
Some derived units are compound units. They are made up of more than one unit. Examples of such units are:
the unit of velocity, 
,
the unit of work/energy, i.e., joule 
,
the unit of density, 
,
the unit of pressure, i.e., pascal 
,
and so on.
A unit of measurement is simply defined as a standard magnitude of a physical quantity that is normally used to express the quantity.
The international system of units (SI units) is the metric system of units generally accepted and used in science all around the world. It is one of the possible units of a quantity.
The standard unit of a quantity can have its submultiples and multiples.
For instance, the SI unit of length is metre (
). Some submultiples of metre are:
A multiple of metre is:
These submultiples and multiples are applicable to other units like seconds, grams, and so on.
Therefore, a unit can be converted to its submultiple or multiple, from one submultiple to another or a multiple, and so on.
For more on unit conversion, see the theory, Unit Conversion.
