The second and the metre have already been successfully defined through fundamental constants. Fixing the numerical value of the hyperfine splitting of caesium defines the second as the duration of 9,192,631,770 periods of the radiation that corresponds to the transition between the two hyperfine levels of the caesium atom. Having defined the second, fixing the numerical value of the speed of light means that the metre is the path length travelled by light in vacuum during a time interval of 1/299,792,458 of a second.

In 2018, the units kilogram, ampere, kelvin and mole will be defined in a similar way. The effect of fixing the numerical value of the Planck constant is a definition of the unit kg m2 s−1 (the unit of the physical quantity called action). Together with the definitions of the second and the metre, this leads to a definition of the kilogram; macroscopic masses can be measured in terms of h, Δν(133Cs) hfs and c. One way of establishing a mass scale is by counting the number of atoms in a silicon single-crystal sphere using the X-ray crystal density (XRCD) approach — probing the regular arrangement of atoms in a perfect lattice — and multiplying it by the known mass of a silicon atom (the 28Si isotope)1. Another route to the kilogram is based on balancing electric and gravitational forces in a so-called watt balance2: in this scheme, the weight of a test mass is compared with the force generated by a coil, the electric power of which is measured very accurately by making use of the Josephson and quantum-Hall effects. The number chosen for the numerical value of the Planck constant will be such that at the time of adopting the definition, the kilogram is equal to the mass of the international prototype currently used for the definition of mass, within the uncertainty of the combined best estimates of the value of the Planck constant at that moment. These estimates are calculated regularly by the CODATA task group on fundamental constants3. (The mission of CODATA, the Committee on Data for Science and Technology, is to improve the quality, reliability, management and accessibility of data of importance to all fields of science and technology.) Subsequently, the mass of the international prototype will become a quantity to be determined experimentally.

The impact of fixing the numerical value of the elementary charge is that the ampere will become the electric current that corresponds to the flow of 1/(1.6021766208 × 10−19) elementary charges per second. Electrical quantities (such as voltage, current and resistance) will be defined by fixing the value of e (and Δν(133Cs) hfs ) instead of the permeability of vacuum μ 0 (which will have an uncertainty equal to that of the fine structure constant α = μ 0 e2c/2h). The conventional (defined) values of the Josephson and von Klitzing constants K J-90 and R K-90 will no longer be needed; these were introduced because they allowed a more precise realization of the electrical units than via today's definition of the ampere. In the future, the numerical values of the Josephson and von Klitzing constants K J and R K will be fixed in terms of the constants e and h (via the relationships K J = 2e/h and R K = h/e2). In particular, the volt and the ohm will be directly realizable, thus making the quantum realizations of the electrical units consistently embedded in the new SI. In fact, there will be a step change in the electrical units realized from quantum standards when the numerical values K J-90 and R K-90 , which have been in use for more than 20 years, are abrogated in favour of the new fixed numerical values.

Fixing the numerical value of the Boltzmann constant means that the kelvin is equal to the change of thermodynamic temperature that results in a change of thermal energy kT by 1.38064852 × 10−23 J. Today's definition of the kelvin is based on an exact temperature value assigned to the triple point of water4. After redefinition, this value will exhibit an uncertainty equal to that of the (currently experimentally determined) Boltzmann constant. With the new definition, it is evident that thermodynamic temperature can be realized directly at any point in the scale without referring to the singular temperature of the triple point of water. At present, thermometry relies on international temperature scales (ITSs), which were developed to give results that are in close agreement with the thermodynamic temperature, and are derived from a series of temperature fixed points (and interpolations between them) that are given conventional values approximating the corresponding thermodynamic temperatures5. Deviations between an ITS — the current ITS was agreed upon in 1990 — and the corresponding thermodynamic temperatures are made available to the user community by the Consultative Committee for Thermometry (CCT). It is expected that the new route with direct traceability to the SI will initially only be used in temperature ranges where primary thermometric methods offer lower uncertainties than ITS-906 (for example, below 20 K and above 1,300 K).