From 2019 onwards, the kilogram will get more accurate. The new artefacts ought to derive from the constants of nature that are all interdependent. The kilogram was the only one among the units still pegged to a real object.

What was the earlier measure of kilogram?
Since 1889, a salt-shaker-sized cylinder, made of 90% platinum and 10% iridium, kept at the Bureau International des Poids et Mesures (BIPM) also called International Bureau of Weights and Measures, Paris and weighing exactly a kilogram served as the definition of the measure. Since 1957 India’s National Physical Laboratory too has a replica of this measure and it has served as the reference.  NPL maintains the National Prototype Kilogram (NPK-57), which is calibrated with International Prototype Kilogram (IPK).

Units of measure no longer linked to physical objects
Several standard units such as second, metre, ampere, Kelvin, mole, candela and, kilogram are no longer defined by physical objects.

For instance, One metre was a platinum-iridium bar of that measure. In 1960, the metre was defined as the distance travelled by light in a vacuum in 1/299,792,458 seconds.

Reason for seprating units from objects
In essence, the units were freed from artefacts, because the artefacts are subject to wear and tear and thus may become a source of error in measurement. Moreover, IPK would put on a little extra mass when tiny dust particles settled on it; when cleaned, it would shed some of its original mass.

New Definition of Kilogram
In 2018, following a vote at BIPM, representatives of 60 countries agreed that the kilogram should be defined in terms of the Planck constant. The Planck constant is a quantity that relates a light particle’s energy to its frequency.

Using a machine called a Kibble balance, in which the weight of a test mass is offset by an electromagnetic force, the value of the Planck constant was fixed, the kilogram was redefined, and the date for the new definition was fixed for May 20, 2019.