 The International Avogadro Coordination ( IAC ) , often simply called the " Avogadro project " , is a collaboration begun in the early 1990s between various national metrology institutes to measure the Avogadro constant by the X @-@ ray crystal density method to a relative uncertainty of 2 × 10 − 8 or less . The project is part of the efforts to redefine the kilogram in terms of a universal physical constant , rather than the International Prototype Kilogram , and complements the measurements of the Planck constant using watt balances . Under the current definitions of the International System of Units ( SI ) , a measurement of the Avogadro constant is an indirect measurement of the Planck constant : 
