The International System of Units, called SI units, is based on seven base units which correspond to seven base physical quantities namely length, mass, time, temperature, amount of substances, light intensity and electric current.

The SI unit of mass is **kilogram**. One kilogram is the mass of a particular cylinder made of Platinum–Iridium alloy, kept at the International Bureau of Weights and Measures in France. This standard was established in 1887 and there has been no change because this is an unusually stable alloy.

Prototype kilograms have been made out of this alloy and distributed to member states. The national prototype of India is the Kilogram no 57. This is preserved at the National Physical Laboratory, New Delhi.

The SI unit of length is **metre**. Earlier the metre (also written as meter) was defined to be 1/10^{7} times the distance from the Equator to the North Pole through Paris. This standard was abandoned for practical reasons. In 1875, the new metre was defined as the distance between two lines on a Platinum-Iridium bar stored under controlled conditions. Such standards had to be kept under severe controlled conditions. Even then their safety against natural disasters is not guaranteed, and their accuracy is also limited for the present requirements of science and technology.

In 1983 the metre was redefined as the distance travelled by light in vacuum in a time interval of 1/ 299792458 seconds. This definition establishes that the speed of light in vacuum is 299792458 metres per second.

The SI unit of time is **second**. The time interval second was originally defined in terms of the time of rotation of earth about its own axis. This time of rotation is divided in 24 parts, each part is called an hour. An hour is divided into 60 minutes and each minute is subdivided into 60 seconds. Thus, one second is equal to 1/86400^{th} part of the solar day.

But it is known that the rotation of the earth varies substantially with time and therefore, the length of a day is a variable quantity, may be very slowly varying.

The XIII General Conference on Weights and Measures in 1967 defined one second as the time required for Cesium-133 atom to undergo 9192631770 vibrations. The definition has its roots in a device, which is named as the atomic clock.

The SI unit of temperature is **kelvin** (K). The thermodynamic scale on which temperature is measured has its zero at absolute zero, and has its lower fixed point corresponding to 273.15 K at the triple point of water (0°C). One unit of thermodynamic temperature (1 K) is equal to 1/273.15 of the thermodynamic temperature of the triple point of water.

The SI unit of electric current is the **ampere** (A). One ampere is defined as the magnitude of current that when flowing through two long parallel wires, each of length equal to 1 m, separated by 1 metre in free space, results in a force of 2 × 10^{–7} N between the two wires.

The SI unit of amount is **mole** (mol). One mole is defined as the amount of any substance, which contains, as may elementary units, as there are atoms in exactly 0.012 kg of C-12 isotope of carbon.

The SI unit of luminous intensity (I) is **candela** (Cd). The candela is defined as the luminous intensity, in a given direction, of a source that emits monochromatic radiation of frequency 540 × 10^{12} hertz and that has a radiant intensity of 1/683 watt per steradian in that direction.