Kilogram gets a new definition

All the world's measurements can be traced back to what's known as SI Units. From just seven base units, all other units of measurement can be derived. Over the years, the definition of the measurements — Ampere, Candela, Kelvin, Kilogram, Meter, Mole, and Second — have been redefined so that they're based on phenomena that occur in nature such as Planck's Constant. The last SI unit based on a physical artefact, the kilogram, has finally been redefined.

Why kill off the kilogram?
Le Grand K has been at the forefront of the international system of measuring weights since 1889. Several close replicas were made and distributed around the globe.
But the master kilogram and its copies were seen to change - ever so slightly - as they deteriorated.
In a world where accurate measurement is now critical in many areas, such as in drug development, nanotechnology and precision engineering - those responsible for maintaining the international system had no option but to move beyond Le Grand K to a more robust definition.

How wrong is Le Grand K?
The fluctuation is about 50 parts in a billion, less than the weight of a single eyelash. But although it is tiny, the change can have important consequences. Coming in is an electrical measurement which Dr Stuart Davidson, head of mass metrology at NPL, says is more stable, more accurate and more egalitarian.
"We know from comparing the kilogram in Paris with all the copies of the kilogram that are all around the world that there are discrepancies between them and Le Grand K itself," he said.
"This is not acceptable from a scientific point of view. So even though Le Grand K is fit for purpose at the moment, it won't be in 100 years' time."

What are the advantages of the new system?
Every few decades, all the replica kilograms in the world had to be checked against Le Grand K. The new system, now that it's been adopted, will allow anyone with a Kibble balance to check their weights anytime and anywhere, according to NPL's Dr Ian Robinson.

Don't worry, you needn't bring your bathroom scale in for calibration because of the new definition. The kilogram won’t change, just the way we define it.

Starting in May 2019, the definition of the kilogram will be:
The kilogram is defined by taking the fixed numerical value of the Planck constant h to be 6.626 070 15 × 10-34 when expressed in the unit J s, which is equal to kg m² s−1, where the metre and the second are defined in terms of c and Δv.