The New SI
Starting May 2019, the definitions of the most of the SI (Système international d'unités) units will change. References to materials (like the triple point of water to define the kelvin of the temperature scale) or references to the artifacts (like the platinum-iridium kilogram prototype) will be eliminated and the seven base units (second, meter, kilogram, ampere, kelvin, mole and candela) will be defined through physical constants. Here is the official document: DRAFT 9th edition of the SI Brochure. The physical constants used for the SI are given by the following.
Here is a table of their current values. The Planck constant, the elementary charge, the Boltzmann constant and the Avogadro constant have inexact (currently experimentally measured) values.
All the following constants will have exact values starting May 20 of this year.
These exact values of the seven constants implicitly define the values of the seven SI base units. These are the seven defining equations.
This defining set of equations can be "solved" for the seven base units and as a result they are expressed through the constants.
Expressing the units through the constants results in integers and rationals with many digits. In 2017, the values of the constants were chosen in such a way that no practically relevant discontinuities in the sizes of the kilogram, the ampere, the kelvin and the mole will occur.
Slightly better to remember are the factored forms of the occurring integers.