In 2018, the kg will be redefined using Planck's constant, rather than the current method of a chunk of metal. This is because the current kg is defined by a series of 'identical' metal chunks. The problem is that these pieces of metal, which are meant to represent an exact kilogram, no longer do and have actually fluctuated, therefore they're no longer perfectly accurate.
See how they plan to redefine the kilo in the video below!
CREDIT TO Veritasium - Youtube (this is not my video)