Fundamental quantities and units

You have read that measurements are concerned with quantities like length, mass, time, density etc. Any quantity which can be measured is called a physical quantity. Out of the different physical quantities, there are seven physical quantities in terms of which other physical quantities can be measured. These fundamental physical quantities are length, mass, time, electric current, temperature, luminous intensity and amount of substance. Such quantities are considered to be the basic or fundamental physical quantities.
If you are asked to measure the quantity of a given amount of milk, you will express the volume of milk in some accepted units of volume. Likewise, if an engineer measures the length of a road that connects two cities, he should express the distance in an accepted unit of length. Such a procedure makes life more comfortable. If there were no common units accepted by all, life would be miserable. Such units are much more essential in scientific measurements to facilitate communication of information at international level.
Any measurement of a quantity includes a reference standard or unit in which the quantity is measured and the number of times the quantity contains that unit. Thus, when we say that the length of a rod is 4 metres, the rod is four times the metre, which is the unit of length. Metre is the standard length that is adopted as a standard for comparison while measuring length. In the process of measurement the accepted reference standard which is used for comparison of a given quantity is called a unit.