In our daily life, we often need to measure various physical quantities. To measure a physical quantity, we compare it with some standard quantity. For example, if we purchase some sugar, we must know how much quantity of sugar we are talking about. Thus, there is a need of some standard quantity for measuring unknown quantity. This standard quantity is called unit.
Various standard units have been in use at different times in different part of the world. With the passage of time. These units were made more precise and acceptable. People especially business communities and scientists of different countries faced problems of converting the units into one another. This problem was solved in a conference of the scientists from all over the world held in Paris.
In 1960, the eleventh general conference of international committee on Weights and Measures recommended that all countries of the world should adopt a system of same kind of standard units. This conference recommended the use of international system of units. It is abbreviated as SI. According to this system, the units of length, mass, time and volume are given in the following table.
| Physical Quantity | Symbol | Unit | symbol |
| Length | L | Meter | M |
| Mass | M | Kilogram | Kg |
| Time | T | Second | S |
| Volume | V | Cubic meter | M3 |
A Practical unit of volume is liter (L). Mostly the liter is used for measuring volume of liquids such as milk, petrol, cooking oil, etc. It is 1/1000th part of cubic meter (M3).
Therefor 1M3 = 1000L
Also 1L = 1000 milliliter= 1000 cubic centimeter (cc)
Prefixes
The main advantage of SI units is that their multiples and sub-multiples can be conveniently expressed using prefixes. Prefixes are based on multiplying and dividing the units by powers of 10. the words or letters added before SI units such as millimeter (m), centimeter © and kilometer (k) are known as prefixes.
- Millimeter means 1000th For example, millimeter (mm) is 1000thpart of a meter, I.e., 1/1000m. it means, 1 m=100 cm.
- Centimeter means 100th For example, centimeter (cm) is 100thpart of a meter, I.e., 1 cm = 1/100m. it means 1 m= 100cm.
- Kilometer means 1000 times. For example, kilometer (km) is 1000 times of a meter, I.e. 1 km =1000m.
Thus, diameter of a thin wire can be written in smaller units of centimeter (cm) or millimeter (mm) instead of meter. Similarly, the longer distance between two cities may be expressed better in a bigger unit of distance, I.e., kilometer (km)