SI units

In that case, metrologists were adapting to fit the needs of chemists, who wanted a way to express SI units on the scale of Avogadro’s number—the 6 × 1023 units in a mole, a measure of the quantity of substances.

In May, it was announced that the international SI units of measurement had been redefined.

New definitions of SI units come into force Revised definitions of four scientific units — the kilogram, the kelvin, the ampere and the mole — came into force on 20 May, World Metrology Day.

The shift marks the biggest shake-up of SI units since 1875, and will be most transformational for the kilogram.

The three other basic SI units — the second, the metre and the candela — are already based on constants.