The SI unit of mass is kilogram and denoted by kg. The standard of mass had not been changed since its establishment which was defined as the mass of a particular alloy of platinum and iridium kept at the International Bureau of Weights and Measures at Severs, near Paris, France.

This definition has its own problems as this is based on an object with mass and it is not universal either and is indeed contaminated easily. It is true that platinum-iridium alloy is unusually stable alloy but it has lost some mass from the date it was created which makes this definition unreliable. We can not think about making our standard that changes.

Prototype of Kilogram based on the old definition.

A change on the standard definition of kilogram was filed and a new definition is established just now to be effective from 2019 may 20. It has been taken as the big achievement on the standard of mass which is now universal and can never change or contaminated. The new definition is based on the fundamental constant called Planck's constant.

The SI unit of Planck's constant is \(\text{Js}\) which is \(\text{kg}\cdot\text{m}^2/\text{s}\). The Plank's constant is the universal constant and it contains second, meter and kilogram. We already have the standard definitions of second and meter and we can use those definitions to define the kilogram. Since we are defining the kilogram in terms of Planck's constant, we must determine the most accurate value of this constant and it's new value is forever taken to be \(6.626 070 15 \times 10^{-34} \text{Js}\).

KILOGRAM: The kilogram is the quantity of matter that gives the fixed value of Planck's constant to be \(6.626 070 15 \times 10^{-34} \text{Js}\).

After this definition we we never need to rely on a piece of material anymore which is prone to contamination. When we need to measure very precisely, the old definition was not enough. The new definition of kilogram took a long time as the Plank's constant must be calculated to a very accurate value. It's value is measured with a machine called Kibble balance.

Kibble balance to measure Planck's constant to a great accuracy.

Now we have the SI unit of mass and we can calculate the mass using this standard such as 1kg, 2kg, 32kg etc.