How do we actually build the new kilogram? Once upon a time the kilogram was a piece of metal preserved in a vault located in a basement. If I needed to build a high precision balance all I needed to do was tuning the the value of my kilograms to that piece of metal.
Things changed in 2019 and the kilogram got redefined. Not anymore exactly equal to a piece of metal preserved in a vault located in a basement, but exactly equal to $\frac{h}{c^2}$ multiplied by a ridiculously large number ($1.356392489652 \times 10^{50}$ hertz).
But I am still a balance manufacturer. How do I tune my kilograms to this exact new value? And how can anyone do it – even allowing some uncertainty?
 A: I have found the answer to my own question: The Kibble balance. For the technical details, see this article.
A: As a balance manufacturer, you’ll still have your standard kilogram lying around, with which you’ll still calibrate your balances. As usual, you’ll send that kilogram to NIST (or your local versions) to itself be calibrated periodically. They’ll tell you the mass of your kilogram to the accuracy available.
The only difference now is how they determine the mass of your kilogram. The NIST Mass and Force Group has yet to implement the new definition for routine calibrations (turns out this is nontrivial). So in practice, nothing has changed; the redefinition is purely conceptual. But in principle you could build your own Kibble balance and thereby accurately calibrate your masses without reference to anyone else (just like you could determine the frequency of Cesium yourself to calibrate the clock division of your business, and then measure the speed of light for your meter stick factory). But, really, who’s going to do that? This is why national metrology institutes exist.
