"In the SI, the standard metre is defined as exactly 1/299,792,458 of the distance that light travels in a second."
"The kilogram was originally defined as the mass of one cubic decimetre of water at 4 °C, standardized as the mass of a man-made artefact of platinum-iridium held in a laboratory in France, which was used until a new definition was introduced in May 2019. Replicas made in 1879 at the time of the artefact's fabrication and distributed to signatories of the Metre Convention serve as de facto standards of mass in those countries. Additional replicas have been fabricated since as additional countries have joined the convention. The replicas were subject to periodic validation by comparison to the original, called the IPK. It became apparent that either the IPK or the replicas or both were deteriorating, and are no longer comparable: they had diverged by 50 μg since fabrication, so figuratively, the accuracy of the kilogram was no better than 5 parts in a hundred million or a proportion of 5x10−8:1. The accepted redefinition of SI base units replaced the IPK with an exact definition of the Planck constant, which defines the kilogram in terms of the second and metre."