Question

why do we need to have a fixed unit for measuring thing ?

Answer 1

Real scalar quantity, defined and adopted by convention, with which any other quantity of the same kind can be compared to express the ratio of the two quantities as a number (International vocabulary of metrology)
Measurement is a process that uses numbersto describe a physical quantity based on what we can observe. This is done to be able to compare them to each other. We can measure how big things are, how warm they are, how heavy they are, and lots of other features as well. Units of Measurementprovide standards for our comparisons, so that the numbers from our measurements refer to the same thing.

For example, the metre is a standard unit to measure length. Before 1982, it was defined as the distance between two markers on a special rod. Now scientists define the metre by using the speed of light. Saying something has a length of 2 metres means that it is exactly twice as long as that rod used to define the metre, or that light takes twice the time defined for a metre to travel that distance.

In past centuries many different units were used in different countries. Today, most units of measure fall into one of three systems:

The older two, the British imperial system and the closely related US customary system use the foot as a measure of length, the pound as a measure for weight, and the second as a measure for time. They use other units as well. The number of smaller units that make the bigger units in these two systems varies: For example, there are 12 inches in a foot and 16 ounces in a pound.

The newest of the three systems is the metric system or SI system which usually use 10, 100 or 1000 of a smaller unit to make a bigger one: For instance, there are 100 centimetres in 1 metre or 1000 grams in 1 kilogram. This system uses the metre for length, thekilogram for weight, and, like the other two systems, the second for time

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