Since 2019, the kg is just defined in terms of the Plank constant and some math with the resonant frequency of cesium as well as the speed of light. There was too much variability in anything physical so they decided to just fix some constants at whatever value they were close to.

https://en.wikipedia.org/wiki/2019_revision_of_the_SI

[–] NegativeInf@lemmy.world 7 points 1 month ago (1 children)

It's part of the attempt to more accurately define Avogadro's number and the kilo.

https://www.nist.gov/si-redefinition/kilogram-silicon-spheres-and-international-avogadro-project

[–] subtext@lemmy.world 5 points 1 month ago* (last edited 1 month ago)

The redefinition of the mole in 2019, as being the amount of substance containing exactly 6.02214076×10^23 particles

Since the 2019 SI redefinition, avogadro’s number is a constant.

https://en.wikipedia.org/wiki/Avogadro_constant

Edit: looks like we were both right! I was reading through your link and it seems the work reported by the NIST led to the exact definitions for Avogadro’s number and the Planck constant.