
Q10: Each of the fundamental constants used to define a unit has an uncertainty; its value is not known exactly. But it is proposed to fix its numerical value exactly. How can you do that? What has happened to the uncertainty?
A10: The present definition of the kilogram fixes the mass of the IPK to be one kilogram exactly with zero uncertainty, u_{r}(m_{IPK}) = 0. The Planck constant is at present experimentally determined, and has a relative standard uncertainty of 4.4 parts in 10^{8}, u_{r}(h) = 4.4 × 10^{−8}.
In the new definition the value of h would be known exactly in the new units, with zero uncertainty, u_{r}(h) = 0. But the mass of the IPK would have to be experimentally determined, and it would have a relative uncertainty of about u_{r}(m_{IPK}) = 4.4 × 10^{−8}. Thus the uncertainty is not lost in the new definition, but it moves to become the uncertainty of the previous reference that is no longer used, as in the table below.
constant used to define the kilogram 
current SI 
New SI 
status  uncertainty 
status  uncertainty 



mass of the IPK, m(K) 
exact  0 
expt  4.4 × 10^{−8} 
Planck constant, h 
expt  4.4 × 10^{−8} 
exact  0 
